Verified answer

Top = 1

Bottom = sec^2(theta)

1/(sec^2theta) = cos^2(theta)

Jen

sin^2 + cos^2 = 1

tan^2 + 1 = sec^2

1/sec^2 = cos^2

(sin²(θ) + cos²(θ)/(1 + tan²(θ)) = 1/(sec²(θ)) = cos²(θ)