SAT Math: Apply the cofunction identity (sin θ = cos(90°−θ))

The concept, explained

• 1

  sin θ = cos(90° − θ) and cos θ = sin(90° − θ). This identity exists because sine and cosine swap roles when angles are complementary.

• 2

  In a right triangle, the two non-right angles are complementary (sum to 90°). The sine of one equals the cosine of the other.

• 3

  Use the identity to convert between sine and cosine when needed — particularly useful when a problem gives you sin of one angle and asks for cos of its complement.

• 4

  For radian form: sin θ = cos(π/2 − θ).