Page 186 - ICSE Math 7

4. Look at this grid paper. Assuming each square to be of dimension 1 cm × 1 cm, calculate the
length of PQ and RS.
Q

S

P R

Pair Activity

Objective: To verify exterior angle property of a triangle

Step 1: Draw two triangles of exactly the same size and shape, one on a sheet of glazed paper
and other on a white drawing sheet. Cut the triangle drawn on the glazed paper.

A A

B C B C

Step 2: Mark the three angles of the triangle drawn on the A
white sheet. Produce one of its side, say BC to D
as shown in the adjacent figure.

∠ACD is called the exterior angle of ∆ ABC formed B C D
at vertex C.
A

Step 3: Now cut out ∠A and ∠B (interior opposite angles

of ∠ACD) from the triangle that is cut from the
glazed paper. B C

A
Step 4: Rearrange the cut outs of ∠A and ∠B on angle ACD
drawn on the white sheet. We find that these two
angles cover ∠ACD entirely and exactly. A
Hence, we conclude ∠ACD = ∠A + ∠B. B B
C D

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