Question

Let θ be an angle in quadrant IV such that sinθ = -2/5 . Find the exact values of secθ and tanθ.

Answer 1

If θ lies in the fourth quadrant, then sin(θ) < 0 and cos(θ) > 0. So we have from the Pythagorean identity,

sin²(θ) + cos²(θ) = 1   ==>   cos(θ) = +√(1 - sin²(θ)) = √21/5

Then

sec(θ) = 1/cos(θ) = 5/√21

and

tan(θ) = sin(θ)/cos(θ) = (-2/5)/(√21/5) = -2/√21