Use Trigonometric functions to find exact value Choose corre

Use Trigonometric functions to find exact value: Choose correct answer.

cos (310*) cos (50*) - sin (310*) sin (50*)

A. 1

B. -1

C. 0

D. sqroot2/2

Solution

given cos (310*) cos (50*) - sin (310*) sin (50*)

we know cos(A + B) = cos A cos B - sin A sin B

let A=310, B= 50 , then

  cos (310*) cos (50*) - sin (310*) sin (50*) = cos(300+50) = cos360 = 1