Q.Find the value of: $\sin150° + \cos120°$.

Both angles are written as $180°$ minus a standard angle, so their values are read from the extended table built via the supplementary-angle rule.

Step 1 — write each angle as $180°$ minus a standard angle.

Step 2 — apply the supplementary-angle rule (sine keeps its sign, cosine flips sign).

$\sin150° = \sin(180°-30°) = \sin30° = \frac12$ …

Forgetting the sign flip for cosine (treating $\cos120°$ as $+\frac12$ instead of $-\frac12$) is the single most common error with obtuse angles — sine never …