# If y=sin θ (theta), what is cos θ in terms of y?

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Use Pythagorean identity to answer this question:

`sin^2(theta) + cos^2(theta) = 1`

Plug in `sin(theta) = y:`

`y^2 + cos^2(theta) = 1`

` cos^2(theta) = 1-y^2`

`cos(theta) = +-sqrt(1 - y^2)`

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