Question

What is $\cos {20}^o+\cos {100}^o+\cos {140}^o$ equal to ?

• A. 2
• B. 1
• C. 1/2
• D. 0

Answer 1

Answer: (D)

0

$\cos {20}^0+\cos {100}^0+\cos {140}^0$

$=2\cos {60}^0\cos {40}^0+\cos {140}^0$ ...... $[\because \cos A+\cos B=2\cos \frac{A+B}{2}\cos \frac{A-B}{2}]$

$=\cos {40}^0+\cos {140}^0$

$=2\cos {90}^0\cos {50}^0$

$=0$ ......... $[\because \cos {90}^o=0]$