38
Subtracting a sum from a number
Textbook: Part 1, lesson 37, pp. 78–79
GoalPractise the rule a − (b + c); a triangle side and equations.
New words
convenient way · qulay usulequation · tenglamatriangle · uchburchakwith equal sides · teng tomonli
Explanation
We choose the convenient way: 77 − (37 + 15) = (77 − 37) − 15 = 40 − 15 = 25, because 77 − 37 gives a round number. The perimeter of a triangle is the sum of three sides: to find an unknown side we subtract the sum of the known sides from the perimeter.
Worked examples
84 − (9 + 64) = (84 − 64) − 9 = 20 − 9 = 11.
A triangle has sides 5 cm and 5 cm and perimeter 14 cm: the third side is 14 − (5 + 5) = 4 cm.
Class activity
“The third side”: children calculate the third side of triangles whose two sides and perimeter are given, then draw and check with a ruler.
Practice
1
77 − (37 + 15) = ?
25
2
98 − (7 + 28) = ?
63
3
A triangle has perimeter 14 cm and two sides of 5 cm. How long is the third side (cm)?
4
4
29 pupils are split into three groups. Group 1 has 9 and group 2 has 12. How many are in group 3?
8