A Level · Cambridge (CIE) · 9709

Integrate positive quadratic reciprocals with inverse tangent

Recognise the inverse-tangent derivative, scale the denominator correctly and check definite accumulation against a positive integrand.

Section 3.5 · Integration

Pure Mathematics 3 · Integration · 3.5

Exam scope and assessment notes

Pure Mathematics 3 (Paper 3, A Level) · Mathematics 9709, 2026–2027, section 3.5. Paper 1 integration, shared standard integrals and identities, Paper 3 partial fractions and derivatives are assumed. Rational decompositions follow the three denominator forms in section 3.1; higher-degree numerators are excluded, while equal-degree cases may need division first. Substitution questions use a given substitution. Trigonometric and inverse-tangent outputs use radians. Written practice is self-checked; human subject-teacher review remains pending.

Before you begin

Differentiate arctan of an inner function.
Reverse a chain multiplier and subtract definite-integral endpoints.

Work on paper. Try each question before revealing help, and explain why your method works.

Quick readiness check

What is d(arctan x)/dx with radian outputs?

What you will learn

Integrate 1/(x² + a²) for a > 0 and scaled positive quadratic reciprocals.
Check the primitive by differentiation and evaluate exact definite integrals in radians.

The blue curve is y = 1/(1 + x²). The shaded region runs from x = 0 to x = 1 above the horizontal axis. The function is positive there, so the signed integral equals the geometric area. The exact area is arctan 1 − arctan 0 = π/4, from inverse-tangent values in radians. The curve continues beyond the plotted window; the drawing illustrates accumulation and does not itself establish an exact numerical area.

Recognise the derivative before choosing a method

Since d(arctan x)/dx = 1/(1 + x²), its reverse gives ∫1/(1 + x²) dx = arctan x + C. For a > 0, differentiation of arctan(x/a) gives (1/a)/[1 + x²/a²] = a/(x² + a²). Therefore ∫1/(x² + a²) dx = (1/a)arctan(x/a) + C. The outside factor and inside scale work together; just replacing 1 by a² in the denominator is insufficient.

For A > 0 and B > 0, ∫1/(A + Bx²) dx = arctan(x√(B/A))/√(AB) + C. Check this by differentiation rather than memorising it without its conditions. Thus ∫1/(2 + 3x²) dx = arctan(x√(3/2))/√6 + C. Every real x is permitted because the denominator stays positive. A difference of squares, such as x² − a², needs a different method and has poles; do not apply the positive-quadratic rule to it.

The integral is accumulation, not a reciprocal of an angle

The inverse notation tan⁻¹ names arctan, not 1/tan. The standard primitive outputs radian values. On 0 ≤ x ≤ 1, 1/(1 + x²) is positive, so its signed integral is also the geometric area beneath it. The exact result is arctan 1 − arctan 0 = π/4. A drawing helps you see its sign and size; exact endpoint values supply the exact answer.

A supplied angular rate θ′(t) = 2/(t² + 4) rad/s integrates to θ(t) = arctan(t/2) + C radians when t is measured numerically in seconds. If θ(0) = π/6, then C = π/6 and θ(2) = 5π/12. The integral of the rate over [0, 2] is the increase π/4, whereas the final angle also includes the initial angle. Use the supplied model and units explicitly.

Pause and explain

Which primitive differentiates to 1/(x² + 4)?

Put the idea to work

Worked example

Find ∫1/(2 + 3x²) dx and verify the coefficient.

Show the worked solution

Write the denominator as 2[1 + (x√(3/2))²].
The inner derivative is √(3/2), so reverse it as well as the outside factor 1/2.
Differentiating arctan(x√(3/2))/√6 gives exactly 1/(2 + 3x²).

Answer arctan(x√(3/2))/√6 + C, for all real x.

Try a topic test

Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.

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From guided practice to a new situation

Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.

1 · Guided

Find ∫5/(x² + 4) dx and check it by differentiation.

Give me a hint

Identify a = 2, then retain the numerator multiplier 5.

Compare my reasoning

The base reciprocal primitive is (1/2)arctan(x/2).
Multiply by 5 and add C to get (5/2)arctan(x/2) + C.
Its derivative is (5/4)/[1 + x²/4] = 5/(x² + 4).

(5/2)arctan(x/2) + C.

Look for these in your work

Scale both the argument and the outside coefficient.
Verify the exact integrand and retain C.

2 · Independent

Evaluate ∫ from 0 to 2 of 6/(x² + 4) dx exactly, and explain the sign.

Give me a hint

A primitive is 3arctan(x/2); use radian endpoint values.

Compare my reasoning

Reverse differentiation gives F(x) = 3arctan(x/2).
F(2) − F(0) = 3(π/4 − 0) = 3π/4.
The integrand is positive throughout, so a positive signed result is consistent.

3π/4.

Look for these in your work

Use exact inverse-tangent endpoint values in radians.
Check the sign against the original integrand.

3 · Transfer

A supplied angular rate is θ′(t) = 2/(t² + 4) rad/s for t ≥ 0, with t measured numerically in seconds. If θ(0) = π/6 radians, find θ(2) and the angular increase from 0 to 2.

Give me a hint

Integrate the rate and determine the constant from the initial angle.

Compare my reasoning

A primitive of the rate is arctan(t/2).
The initial condition gives θ(t) = arctan(t/2) + π/6.
At 2 s the angle is π/4 + π/6 = 5π/12 radians; its increase is π/4 radians.

Final angle 5π/12 radians; increase π/4 radians.

Look for these in your work

Match the primitive to the supplied angular-rate units.
Distinguish initial angle, increase and final angle.

Common mistakes

Keep both the outside coefficient and the scale inside arctan.
Use the rule only for the stated positive quadratic form.
An integrated rate gives an increase; include an initial value separately.

Recall without your notes

What can you explain now?

Find ∫1/(x² + 9) dx.

Compare with the explanation

(1/3)arctan(x/3) + C.

The derivative is (1/9)/[1 + x²/9] = 1/(x² + 9).

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Make it stick

Derive the scaled positive-quadratic primitive from the inverse-tangent derivative, then check it and evaluate an exact definite integral.

If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.

Keep learning Mathematics

Check the official syllabus 2026–2027

Course scope and assessment

312 sequenced lessons provide 936 guided, independent and application problems covering the 160 objectives in sections 1.1–1.8, 2.1–2.6, 3.1–3.9, 4.1–4.5, 5.1–5.5 and 6.1–6.5. The map lists all 38 sections across six alternative components; all eight Pure Mathematics 1 sections and all six Pure Mathematics 2 sections, from Paper 2 Algebra through Numerical solution of equations, currently have full teaching sequences. Paper 3 Algebra, section 3.1, links the six shared foundation lessons plus six lessons on partial fractions and rational binomial expansions; Paper 3 logarithms, trigonometry and numerical equations use the shared lessons, with the Paper 1 graph foundation linked into trigonometry. Paper 3 differentiation combines the shared methods with its inverse-tangent derivative lesson. Paper 3 Integration links four shared standard and identity lessons plus seven advanced lessons, covering all six section 3.5 requirements. Eight Vectors lessons cover all six section 3.7 requirements, with 2D and 3D line and solid applications. Nine Differential equations lessons cover all four section 3.8 requirements, including Paper 3 integration techniques, initial conditions, equilibrium solutions and conditional model interpretations. Eleven Complex numbers lessons cover all eight section 3.9 requirements, with seven Argand diagrams and explicit argument and boundary checks. All nine Pure Mathematics 3 sections now have teaching linked to every mapped requirement. Nine Forces and equilibrium lessons cover all seven section 4.1 requirements, with six force diagrams, signed incline-friction intervals, reaction-dependent pull and push limits and separate-body third-law checks. Nine Kinematics lessons cover the four official section 4.2 requirements, with the graph requirement mapped to three relationships. Six motion diagrams support directed quantities, graph slopes and signed areas, Paper 1 time-calculus methods, reversal distances, constant-acceleration stages and shared-clock particle meetings. Six Momentum lessons cover both section 4.3 requirements, including signed system totals, conservation assumptions, outgoing velocities, coalescence, unknown masses and successive impacts. Four diagrams show system momentum through the stipulated events, and supplied kinetic-energy checks distinguish conservation of momentum from changes in kinetic energy. Twelve Newton’s laws lessons cover all four section 4.4 requirements, with vertical support and release stages, rough horizontal and incline motion, inferred parameters, hanging and surface pulley systems, rigid tow-bar thrust and slack-string transitions. Seven force diagrams support the separate-particle balances. Nine Energy, work and power lessons cover all five section 4.5 requirements, with angle-dependent work, kinetic and reference potential energy, signed net-work balances, smooth curved slides, rough and driven slopes, ideal connected-system energy and instantaneous power on hills. Five diagrams distinguish the energy inputs, losses and valid positive-speed force model. All five Mechanics sections now have teaching and staged practice linked to their mapped requirements. Thirteen Representation of data lessons cover all five section 5.1 requirements, including choice and limitations of displays, ordered and back-to-back stems, raw summaries and box plots, unequal-width density histograms, cumulative-frequency interpolation, descriptive moment calculations, grouped estimates, coding, combining two datasets and correcting totals. Six accessible diagrams retain repeated observations and distinguish numerical spread, frequency area and estimated readings. Ten Permutations and combinations lessons cover both official section 5.2 requirements, with selection and ordered roles, repeated objects, adjacency blocks, non-adjacency gaps, leading-zero cases, labelled multi-row seating and mixed restrictions. Six accessible diagrams show outcome trees, ordered versus unordered choices, repeated copies, blocks, gaps and distinct row seats. Eleven Probability lessons cover all four section 5.3 requirements, with equiprobable elementary outcomes, selection-based probabilities, complements and disjoint regions, independence tests, restricted sample spaces, two-way tables, replacement and removal trees, reversed conditions and unknown parameters. Seven accessible diagrams show dice outcomes, random subsets, event regions, a conditioned grid, a count table and two probability trees. Twelve Discrete random variables lessons cover all three official section 5.4 requirements, with complete probability tables, moment and parameter checks, binomial assumptions, exact probabilities and moments, geometric first-success probabilities, tails, expectation and integer reliability thresholds. Six accessible diagrams distinguish outcome grouping, weighted moments, fixed trials, discrete probability bars, waiting conventions and integer tail deadlines. Twelve Normal distribution lessons cover all three official section 5.5 requirements, with continuous-area interpretation, MF19 cumulative tables, full standardisation, tail and interval regions, conditional probabilities, percentiles, one- and two-parameter inference, binomial suitability, continuity corrections and approximate integer thresholds. Seven accessible diagrams show model areas, table reading, aligned scales, intervals, quantiles, matched binomial and normal shapes and whole integer cells. All five Statistics 1 sections now have teaching and staged practice linked to their mapped requirements. Fourteen Poisson distribution lessons cover all five official section 6.1 requirements, with homogeneous event-count assumptions, exposure scaling, exact masses and tails, conditional count events, moments, parameter inference, probability duration targets, disjoint and overlapping windows, rare-count binomial approximations, matched normal approximations and continuity corrections. Nine accessible diagrams show event exposures, unit conversions, discrete bars, complete tail complements, population moments, shared windows, approximate count models and integer cells. Exact checks distinguish approximate and exact close-target decisions. Fourteen Linear combinations lessons cover the six listed results in official section 6.2, with affine moments, signed scales and constant cases, expectation without independence, independent squared variance contributions, fixed and random fees, independent copies versus reused values, affine normal events, independent normal sums and differences, weighted and conditional areas, coefficient inference, exact independent Poisson sums and overlapping exposure limits. Nine accessible diagrams distinguish scale changes, variance contributions, component costs, copy mechanisms, aligned normal scales, summed areas, Poisson count pairs and unsupported distribution claims. Fourteen Continuous random variables lessons cover both official section 6.3 requirements, with density validity and units, support-aware intervals, complements and conditional regions, first and second moments, percentiles by direct density areas, symmetry checks, parameter inference, infinite-support power and exponential integrals, divergent-moment limits and combined applications. Nine accessible diagrams distinguish density height from probability, normalisation, negative-density failures, central and conditional areas, mean versus median, percentile locations and partial infinite tails. Explicit cumulative distribution function knowledge is not required. Eighteen Sampling and estimation lessons cover all eight official section 6.4 requirements, with population and frame definitions, random-number selection, sampling-quality limits, independent sample-mean moments, normal means, CLT conditions, SE scaling, raw, summarised and coded unbiased estimates, known-variance and large-sample mean intervals, repeated-sampling interpretation, precision planning, proportion intervals, sparse-outcome limits and contextual applications. Ten accessible diagrams distinguish selection frames, number rejection, repeated sample means, population versus mean density, CLT models, variance divisors, interval targets, illustrative coverage, integer size checks and proportion margins. Eighteen Hypothesis tests lessons cover all five official section 6.5 requirements, with pre-specified alternatives, inclusive count tails, explicit equal-tail conventions, integer critical regions and actual significance, exposure-aware Poisson tests, continuity-corrected count approximations, normal and large-sample mean tests, confidence-interval connections, error classifications, fixed-region binomial, Poisson and normal error probabilities and sample-size power comparisons. Nine accessible diagrams show the testing procedure, exact count regions, unequal Poisson tails, discrete significance jumps, corrected count boundaries, matching mean intervals and alternative error distributions. All shared lessons are counted once in the course total. All 38 sections now link teaching and practice. Eight earlier overview lessons remain available, but do not count as full section coverage. This is a content-availability measure, not full programme completion or exam readiness. Written practice is self-checked, not officially marked. Check the 2028–2030 syllabus separately if sitting an examination from 2028; this teaching map uses 2026–2027 requirements.