Question

Sum and Difference Identities

Find the value of the expression.
cos(-pi/12)
What would -pi/12 be the sum or difference of?

Answer 1

Notice that

1/6 - 1/4 = -1/12

Then

cos(-π/12) = cos(π/6 - π/4)

cos(-π/12) = cos(π/6) cos(π/4) + sin(π/6) sin(π/4)

cos(-π/12) = (√3/2) (1/√2) + (1/2) (1/√2)

cos(-π/12) = (√6 + √2)/4