Question

How to solve this question without using calculator?

cos100+cos440

Answer 1

0

Explanation:

using the addition formula for cos

cos(a ± b ) = cosacosb ∓ sinasinb)

given

cos100 + cos440

= cos(180 - 80) + cos(360 + 80)

= cos180cos80 + sin180sin80 + cos360cos80 - sin360sin80

= (- 1)cos80 + 0(sin80) + (1)cos80 - 0(sin80)

= - cos80 + 0 + cos80 - 0

= 0