2.5 Graph of Functions (II), SPM Paper 2 (Long Questions)


Question 9:
(a) Complete the table in the answer space for the equation y = 8 – 3x – 2x2 by writing down the values of y when x = –4 and x = 2.

(b) For this part of the question, use graph paper. You may use a flexible curve rule.
By using a scale of 2cm to 1 unit on the x-axis and 2cm to 5 units on the y-axis, draw the graph of y = 5 – 8x – 2x2 for –5 ≤ x ≤ 3 and –27 ≤ y ≤ 9.

(c) From your graph, find
(i) the value of y when x = –2.5,
(ii) the positive value of x when y = 16.

(d) Draw a suitable straight line on your graph to find the values of x which satisfy the equation 8 – 3x – 2x2 = 0 for –5 ≤ x ≤ 3 and –27 ≤ y ≤ 9.

Answer:

x
–5
–4
–3.5
–2
–1
0
1
2
3
y
–27
r
–6
6
9
8
3
s
–19
Calculate the value of r and s.

Solution:
(a)
y = 8 – 3x – 2x2  
when x = –4,
r = 8 – 3(–4) – 2 (–4)2
= 8 + 12 – 32 = –12

when x = 2,
s = 8 – 3(2) – 2(2)2
= 8 – 6 – 8 = –6

(b)


(c)
(i) From the graph, when x = 2.5, y = 2.5
(ii) From the graph, when y = 16, positive value of x = 2.8

(d)
y = 8 – 3x – 2x2 —– (1)
0 = 5 – 8x – 2x2  —– (2)
(1) – (2) : y = 3 + 5xy = 5x +3
The suitable straight line is y = 5x +3.

Determine the x-coordinates of the two points of intersection of the curve y = 8 – 3x – 2x2 and the straight line y = 5x +3.

x
–5
0
y = 5x + 3
22
3
From the graph, x = –4.5, 0.55.

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