Which trig expression is equal to sin (72° - a)?
cos 18°
sin 18°
cos (90° - (72° - a))
sin (90° - (72° - a ))

Solution:

Given sin (72° - a)

From complementary angle property of trigonometry,

We know that sinθ = cos(90 - θ)

Here θ = 72 - a

So, sin(72° - a) = cos(90 - (72° - a))

Therefore, trigonometric expression is equal to sin (72° - a) = cos(90 - (72° - a))

Which trig expression is equal to sin (72° - a)?

Summary:

Trigonometric expression is equal to sin (72° - a) = cos(90 - (72° - a)).