Page 18 - 新思维数学活动用书8 样章

2.2 Using expressions and formulae

d 10 − x (Work out the division before the subtraction)
3
x
10 − = 10 − 6
3 3
= 10 −
=

e 2(x + 4) (Work out the bracket before the multiplication)
2 × (x + 4) = 2 × (6 + 4)
= 2 ×

=

4 Work out the value of the expression
a a + 3 when a = 7 b b + 6 when b = −4
c 3c when c = −3 d d when d = −35
5
e a + b when a = 3 and b = −5 f c − d when c = 14 and d = 7

g 5e + 2f when e = 3 and f = 5 h 2(g + h) when g = 9 and h = −20
i 3x − 7 when x = −5 j 10(9 − x) when x = 6

k x − 10 when x = 20 l x + y when x = 30 and y = −30.
4 2 10

5 Use the formula H = kv to work out the value of H when
a k = 9 and v = 3 b k = 8 and v = −2.

Practice

6 Work out the value of the expression
a a − 6 when a = 4 b 30 − b when b = 6
2
2
c a + b when a = 3 and b = 4 d c − d when c = 5 and d = 6
2
2
2
2
e 3p when p = 4 f 5q + 1 when q = 10
2
2
3
3
g t when t = 2 h 10v when v = 4 Tip
s 3
3
i z − 2 when z = 2 j when s = 10 10v =
3
10
k 2(m + 3) when m = −4 l 5(n + 10) when n = −2. 10 × v × v × v
3
2
7 a Write a formula for the number of seconds in any number of minutes,
using
i words ii letters.
b Use your formula from part a to work out the number of seconds in 30
minutes.
8 Use the formula d = 16t + 38 to work out d when t = 2.

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