Sine of 90 degrees minus theta equals cosine of theta, and cosine of 90 degrees minus theta equals sine of theta. sin(30°) = cos(60°). The short list of formulas is in ACT Math formulas.
The two acute angles in a right triangle
Those two angles add to 90, so the sine of one is the cosine of the other. If a choice says sin(90° − θ) and another says cos θ, they match. Tangent and cotangent swap the same way.
Reciprocal identities are a different family, in reciprocal trig identities. Values to memorize are in trig values to memorize. Right-triangle setup is in right-triangle trigonometry.
The bottom line
sin(90° − θ) = cos θ. cos(90° − θ) = sin θ.
Start practicing
Start with a free diagnostic, then drill your weak spots with 15-question quizzes and track how you're doing across Reading, English, and Math. Compare plans whenever you're ready to go further.
This article offers general ACT prep guidance. The ACT can change from year to year, including its format, scoring, policies, test dates, and fees, so always confirm the latest details on the official ACT website at act.org before you make decisions. ACT® is a registered trademark of ACT, Inc. thirty-six is not affiliated with, endorsed by, or sponsored by ACT.