(1-sin²a).tan²a ekuivalen dengan

1 - sin²a = cos²a
maka
(1 - sin²a) tan²a
= (cos²a) sin²a/cos²a
= sin²a

*cos^2 a + sin^2 a = 1 <=> cos^2 = 1 - sin^2 a
*tan a = sin a / cos a <=> tan^2 a = sin^2 a / cos^2 a

Maka
(1 - sin^2 a).tan^2 a
= cos^2 a × sin^2 a / cos^2 a
= sin^2 a