Page 36 - 新思维数学活动用书7 样章

2.5 Constructing and solving equations

7 Write an equation for each of these statements. Solve each equation to find the value of the
unknown number.
a b

I think of a number I think of a number
and then add 5. My and then subtract
answer is 21. What 5. My answer is 21.
is the number I first What is the number I
thought of? first thought of?

8 Write an equation for each of these statements. Solve each equation to find the value of the
unknown number.
a I think of a number and add 14. The answer is 20.

b I think of a number and subtract 17. The answer is 20.
c I think of a number and multiply it by 5. The answer is 20.
9 Solve each of these equations. Check your answers by substituting your answer back into the
original equation.

a 2a + 4 = 18 b 5a − 2 = 18 c 30 = 7b + 9 d 18 = 6b − 12

Challenge

10 This is part of Sofia’s homework.

Question
The diagram shows a right angle divided into two smaller
angles, 3x and 2x.
Work out the size of the angles in this diagram. 3x
2x
Solution
Equation 3x + 2x = 90°
Simplify 5x = 90°
90
Solve x = = 18°
5

Answers 3x angle is 3 × 18 = 54°.
2x angle is 2 × 18 = 36°.

Check 54 + 36 = 90° ✓

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