Page 38 - 新思维数学学生用书8 样章

2.2 Using expressions and formulae

3 Work out the value of each expression. Tip

a x + 5 when x = 4 b 10 − y when y = 5
2
2
3
c g + h when g = 3 and h = 6 d m − n when m = 7 and n = 8 Remember that r
2
2
2
2
e 4k when k = 2 f 3r when r = 1 means r × r × r.
3
2
g 2y when y = 3 h x − 5 when x = 2
3
3
i 20 − w when w = 4 j y 2 when y = 4
3
2
4 This is part of Dakarai’s homework. He has made a mistake in his working.
Question
Work out the value of x – 8 when x = –3.
2
Answer
x – 8 = (–3) – 8
2
2
= –3 × –3 – 8
= –9 – 8

= –17

a Explain the mistake he has made.
b Work out the correct answer.
c Work out the value of y + 4 when y = −5.
2
5 This is part of Oditi’s homework. She has made a mistake in her working.

Question
Work out the value of 5x when x = –2.
3
Answer
5x = 5 × (–2) 3
3
= (–10) 3
= –10 × –10 × –10

= –1000

a Explain the mistake she has made.
b Work out the correct answer.

c Work out the value of 2y when y = −3.
3
6 a Write a formula for the number of months in any number of years, in

i words ii letters.
b Use your formula in part a ii to work out the number of months in 8 years.

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