Page 38 - 新思维数学学生用书7 样章

2.2 Using expressions and formulae

Worked example 2.2

a Work out the value of these expressions.

i a + b when a = 2 and b = 4
ii 2w − 3v when w = 12 and v = 5
b Write a formula for the number of days in any number of weeks, in:
i words ii letters
c Use the formula in part b to work out the number of days in eight weeks.

Answer
a i a + b = 2 + 4 Substitute 2 for a and 4 for b in the expression.

= 6 Add 2 and 4 together to give a total of 6.
ii 2w − 3v = 2 × 12 − 3 × 5 Substitute 12 for w and 5 for v in the expression.
= 24 − 15 Use the correct order of operations and do the

= 9 multiplications before the subtraction.
b i number of days = There are seven days in a week, so multiply the
7 × number of weeks number of weeks by 7.

ii d = 7w Choose d for days and w for weeks and write
7 × w as 7w.
Always write the number before the letter, so
write 7w not w7.
c d = 7 × 8 Substitute w = 8 into the formula.

= 56

Exercise 2.2

1 Copy and complete the workings to find the value of each expression.
a x + 9 when x = 13 b x when x = 40
5 Tip
x + 9 = 13 + 9
x = 40 x means x ÷ 5.
= 5 5 5
=

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