Which relationship in the triangle must be true?

sin (B) = sin (A)

sin (B) = cos (90 - B)

cos (B) = sin (180 - B)

COS (B) = COS (A)

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sin (B) = cos (90-B)

Step-by-step explanation:

sin (B) = cos (90-B) is a co-function identity.

We can also prove it using the difference identity for cosine.

Let's do that:

cos (90-B)

equals

cos (90) cos (B) + sin (90) sin (B)

0cos (B) + 1sin (B)

0+sin (B)

sin (B)

Therefore cos (90-B) = sin (B).