Question

What is the measure (in degrees) of angle $A$?

• A. $53.4$
• B. $58.7$
• C. $63.6$
• D. $70.1$
• E. $80.3$

Answer 1

The correct option is E $80.3$

From figure, we have

$\cos A=\frac{{10}^2+{13}^2-{15}^2}{2\times 10\times 13}$

$=\frac{100+169-225}{260}$

$=\frac{44}{260}$

$=\frac{11}{65}$
Therefore $A={\cos }^{-1}\left(\frac{11}{65}\right)=80.3$