Question

ΔABC has three sides with these lengths: AB = 9, BC = 40, and CA = 41. What is the value of cos C? A. `9/41` B. `40/9` C. `40/41` D. ` 9/40`

Answer 1

C

using the cosine rule

cosC = ( a² + b² - c² ) / (2ab )

with a = 40, b = 41 and c = 9, then

cosC = ( 40² + 41² - 9² ) /( 2× 40 × 41 ) = $\frac{3200}{3280}$ = $\frac{40}{41}$ → C