Answer:
The unknown side to the triangle is 75 cm.
Explanation
Let x be the length of the unknown side:
x^2 + 40^2 = 85^2
x^2 + 1600 = 7225
x^2 = 7225 - 1600
x^2 = 5625
x = 75
75 is the length of the third side
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