A Maths Revision Notes >>

Cosine graph

Shape and features of graph

$$ \text{General equation: } y = a \cos bx + c $$


Shape and features when a > 0 (click to show):



Shape and features when a < 0 (click to show):



Questions

Deduce equation from graph

Q1. The diagram below shows the graph of $y = a \cos (bx) + c $.

diagram

Determine the values of $a$, $b$ and $c$.

(from A Maths 360 Workbook Ex 11.3)

Answer: $ a = 3, b = 2, c = 0 $

Solutions


Q2. The diagram below shows the graph of $y = a \cos (bx) + c $.

diagram

Determine the values of $a$, $b$ and $c$.

(from A Maths 360 Workbook Ex 11.3)

Answer: $ a = -2, b = 4, c = 2 $

Solutions


Deduce equation from information

Q3. The curve $y = a \cos \left(x \over b\right) + c$, where $a$ and $b$ are positive integers, has an amplitude of $4$ units and period of $720^\circ$. The maximum value of $y$ is $3$. State the values of $a$, $b$ and $c$.

Answer: $ a = 4, b = 2, c = -1 $

Solutions


Sketch question

Q4. Sketch the graph of $y = -1 - \cos \left({x \over 2}\right)$, for $ 0 \le x \le 6 \pi $.

Solutions


Symmetry question

Q5. The diagram above shows part of the graph of $y = 6 - 3 \cos 2x$, passing through the points $(k, 4)$, $(l, 8)$ and $(m, 4)$, where $k$, $l$ and $m$ are constants.

diagram

Using symmetry of the graph, or otherwise, find an equation connecting

(from think! A Maths Worksheet 9B)

(i) $\pi$, $k$ and $m$,

Answer: $ m = k + \pi $

Solutions

(ii) $\pi$, $k$ and $l$,

Answer: $ k + l = {\pi \over 2} $

Solutions


Deduce number of solutions to equation

Q6(i) On the same axes, sketch the graphs of $y = 2 \cos 2x$ and $y = \sin x + 1$ for $0 \le x \le 2\pi$.

(from A Maths 360 2nd edition Ex 10.2)

Solutions

Q6(ii) Hence, find the number of distinct values of $x$, in the interval $0 \le x \le 2\pi$, for which $\cos 2x - {1 \over 2} = {1 \over 2} \sin x $.

Answer: $ \text{4 distinct values of } x $

Solutions


Past year O level questions

Year & paper Comments
2024 P1 Question 10 Sketch graph and deduce solutions to equation
2023 P2 Question 7a Deduce equation from information provided (difficult)
2022 P1 Question 10 Sketch graph
2017 P2 Question 10 Sketch graph and deduce number of solutions to inequality
2016 P1 Question 3b Deduce equation from graph
2014 P2 Question 9 Sketch graph and deduce solutions to equation
2012 P1 Question 8 Deduce equation from information and sketch graph
2010 P2 Question 11i Deduce equation from information and sketch graph
2008 P2 Question 7 Sketch graph
2007 P1 Question 9 Sketch graph and deduce solutions to equation
2004 P1 Question 6 Find amplitude, period and the coordinates of maximum point and minimum point (Link - Subscription required)


Trigonometry: Sine graph Trigonometry: Tangent graph