Q.Find the value of: $\sin^2120° + \cos^2150°$.

Both terms are standard obtuse-angle values, squared — the negative sign on $\cos150°$ disappears once it is squared.

Step 1 — look up the two obtuse-angle values.

Step 2 — square each (the negative sign on $\cos150°$ is removed by squaring). …

Carrying the negative sign through the squaring step (writing $\cos^2150°=-\frac34$) is a common arithmetic slip — a negative number squared is …