Question

Write the trigonometric expression as an algebraic expression in u. cos (sin-1 u)

Answer 1

cos(arcsin(u)) = √(1 - u²)

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You want cos(α) where sin(α) = u.
You know that
cos(α)² + sin(α)² = 1
cos(α)² = 1 - sin(α)² = 1 - u²
cos(α) = √(1 - u²)