Three ratios run this whole chapter, and one word remembers them all: SOH CAH TOA. $\sin\theta = \dfrac{o}{h}$ — sine theta equals opposite over hypotenuse. $\cos\theta = \dfrac{a}{h}$ — cosine theta equals adjacent over hypotenuse. $\tan\theta = \dfrac{o}{a}$ — tangent theta equals opposite over adjacent. The little letters are just first initials, and they are all side LENGTHS in the same unit: $o$ is the opposite, $a$ the adjacent, $h$ the hypotenuse — the three names you dealt out last lesson. Their full names are sine, cosine and tangent — sin, cos and tan on your calculator.

Choosing between them isn’t a talent, it’s a checklist: mark the angle, then name the two sides the story actually stars — the one you know and the one you want — and pick the ratio built from exactly that pair. The side the problem never mentions is the tell: no hypotenuse anywhere means tangent; no opposite means cosine; no adjacent means sine.