Sketch trigonometric graphs and track their transformations
Use periods, ranges, key points and tangent asymptotes to sketch graphs in degrees or radians.
Pure Mathematics 1 (Paper 1) · AS & A Level Mathematics 9709, 2026–2027. State angle units and find every solution in the supplied interval; general forms of solution are not required in section 1.5. Written practice is self-checked. This sine, cosine and tangent graph foundation also supports Pure Mathematics 3 (Paper 3, A Level), section 3.3, alongside the shared reciprocal-function lessons.
Before you begin
- Convert degrees and radians.
- Understand function graph transformations and an excluded input.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the radian period of sin x?
What you will learn
- Sketch and use sine, cosine and tangent for angles of any size.
- Apply vertical scaling, horizontal scaling, shifts and reflections to trigonometric graphs.
Build a sketch from landmarks and repetition
Sine and cosine are defined for every real angle, have range −1 ≤ y ≤ 1 and repeat after 360° or 2π radians. In one radian period, sin x takes values 0, 1, 0, −1, 0 at x = 0, π/2, π, 3π/2, 2π. Cosine takes 1, 0, −1, 0, 1 at those inputs. Repeat the pattern for negative inputs or further turns; label the horizontal axis with its angle unit.
Tangent has period 180° or π radians and takes every real output. It is undefined when cos x = 0: for example at π/2 and 3π/2. These are vertical asymptotes, not points to join across. Within −π/2 < x < π/2, tan x increases through (0, 0) and becomes unbounded towards either asymptote. Tangent does not have the bounded range or amplitude of sine and cosine.
Transform the input before applying the output change
For y = a sin(bx) with b ≠ 0, the amplitude is |a| and the radian period is 2π/|b|. A vertical shift changes the midline but not the period. For y = 1 − cos 2x, first halve the cosine period to π, then reflect vertically and shift up 1. Its range is 0 ≤ y ≤ 2, with minima at 0, π, 2π and maxima at π/2, 3π/2 in the displayed interval.
For y = tan(x + π/4), shift the tangent graph left π/4. On −π/2 < x < π/2 it has a zero at −π/4 and an asymptote at π/4, with separate branches on either side. A shift inside brackets has the opposite direction from its written sign. Mark asymptotes before plotting and never draw a continuous curve through an excluded input.
Pause and explain
What is the range of y = 1 − cos 2x?
Worked example
Describe and sketch y = 1 − cos 2x for 0 ≤ x ≤ 2π.
Show the worked solution
- The input factor 2 halves the cosine period from 2π to π.
- The negative sign reflects the output and +1 shifts its midline to y = 1, so its range is [0, 2].
- Plot minima at (0, 0), (π, 0), (2π, 0) and maxima at (π/2, 2), (3π/2, 2), joining smoothly.
Answer Period π; range 0 ≤ y ≤ 2; minima at x = 0, π, 2π and maxima at x = π/2, 3π/2.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →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.
For y = 3 sin x on 0 ≤ x ≤ 2π, give five quarter-period points, its amplitude and period.
Give me a hint
Scale the standard sine outputs by 3 while retaining the original input positions.
Compare my reasoning
- Use inputs 0, π/2, π, 3π/2 and 2π with sine outputs 0, 1, 0, −1 and 0.
- Multiply each output by 3 to get (0, 0), (π/2, 3), (π, 0), (3π/2, −3), (2π, 0).
- The greatest distance from the midline is 3 and the pattern repeats after 2π.
Points (0, 0), (π/2, 3), (π, 0), (3π/2, −3), (2π, 0); amplitude 3; period 2π.
Look for these in your work
- Scale outputs without changing the input positions.
- Distinguish amplitude from period.
Describe the range, period and turning points of y = 1 − cos 2x on 0 ≤ x ≤ 2π.
Give me a hint
First locate where cos 2x is 1 or −1; the reflection reverses which is a maximum.
Compare my reasoning
- The period is 2π/2 = π and the range is [0, 2].
- cos 2x = 1 at x = 0, π, 2π, giving minimum output 0.
- cos 2x = −1 at x = π/2, 3π/2, giving maximum output 2.
Period π; range [0, 2]; minima (0, 0), (π, 0), (2π, 0); maxima (π/2, 2), (3π/2, 2).
Look for these in your work
- Account for the horizontal factor and both output transformations.
- Identify every extremum in the stated interval.
For y = tan(x + π/4) on −π/2 < x < π/2, identify its zero, vertical asymptote and period. Explain how to draw its branches.
Give me a hint
Shift the standard tangent input landmarks left π/4.
Compare my reasoning
- The permitted zero occurs when x + π/4 = 0, so x = −π/4.
- The only asymptote in this interval occurs at x + π/4 = π/2, so x = π/4.
- The period remains π; sketch two increasing branches separated at π/4 and do not assign a value there.
Zero x = −π/4; asymptote x = π/4; period π; separate increasing branches.
Look for these in your work
- Move input landmarks in the correct direction.
- Represent the excluded asymptote instead of joining across it.
Common mistakes
- Label whether the input axis is in degrees or radians.
- A horizontal input factor changes period, rather than amplitude.
- Tangent is undefined at its vertical asymptotes; do not connect the branches across them.
What can you explain now?
Give the amplitude and period of y = 3 sin x using radians.
Compare with the explanation
Amplitude 3; period 2π.
The output factor scales the vertical values only; the unscaled input keeps the sine period.
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Make it stick
Sketch one period of sine, cosine and tangent from memory. Explain how an output multiplier, input multiplier and input shift affect each sketch.
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.