Question

In a right triangle, one angle measures x°, where sin xº 4/5 What is cos(90° - xº)?

Answer 1

Answer:   4/5

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Rule:

if A+B = 90, then cos(A) = sin(B) and sin(A) = cos(B)

This can be rephrased into cos(A) = sin(90-A) and sin(A) = cos(90-A)

We'll focus on the second case sin(A) = cos(90-A)

Simply replace A with x and we have the same idea.

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So,

sin(x) = cos(90-x)

cos(90-x) = sin(x)

cos(90-x) = 4/5

You can prove this by drawing out a right triangle as shown below.