Let’s denote by x the length of the third side of this triangle.
According to the condition of the problem, the two sides of this triangle are 5/8 cm, and the perimeter of this triangle is 21/16 cm.
The perimeter of any triangle is equal to the sum of the lengths of all three sides of this triangle, therefore, we can draw up the following equation:
5/8 + 5/8 + x = 21/16.
We solve the resulting equation and find the length of the third side of this triangle:
10/8 + x = 21/16;
x = 21/16 – 10/8;
x = 21/16 – 20/16;
x = 1/16 cm.
Answer: the length of the third side of this triangle is 1/16 cm.
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