WAEC - Further Mathematics (2021 - No. 24)
If sin x = \(\frac{12}{13}\) and sin y = \(\frac{4}{5}\), where x and y are acute angles, find cos (x + y)
\(\frac{48}{65}\)
\(\frac{13}{15}\)
\(\frac{-33}{65}\)
\(\frac{-48}{65}\)
Maelezo
From Pythagoras' theorem
when sin x = \(\frac{12}{13}\), cos x = \(\frac{5}{13}\)
Using cos (x + y) = cosx cosy - sinxsiny
\(\frac{5}{13}\) * \(\frac{3}{5}\) - \(\frac{12}{13}\) * \(\frac{4}{5}\)
= \(\frac{-33}{65}\)