The sum of the legs of a right triangle is 17 an hypothenuse is 13. What is the product of cosines of acute angles?

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giorgiana1976 | College Teacher | (Level 3) Valedictorian

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Let b and c be the legs of the right angle triangle and a is the hypothenuse.

According to enunciation, the sum of the legs is:

b + c = 17

The hypotenuse is of 13 units: a = 13

Let B and C be the acute angles of the right triangle.

cos B = adjacent leg/hypotenuse = c/a

cos C = b/a

We'll multiply cos B by cos C:

cos B*cos C = c*b/a^2

We'll apply Pythagorean theorem in a right angle triangle:

a^2 = b^2 + c^2

But b^2 + c^2  = (b+c)^2 - 2bc

a^2 = (b+c)^2 - 2bc

13^2 = 17^2 - 2bc

We'll isolate 2bc to the left:

2bc = 17^2 - 13^2

2bc = (17 - 13)(17 + 13)

bc = 4*30/2

bc = 60

cos B*cos C = bc/a^2 = 60/169

The requested value of the product cos B*cos C is: cos B*cos C = 60/169.

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