cos/pi question?

Question

how does 5/12pi = cos( pi/6 + pi/4)????

Answer 1

Do you mean cos(5/12pi) = cos( pi/6 + pi/4)?
Reason:
5/12 = (2+3)/12 = 1/6 + 1/4

Answer 2

Using the trig identity
Cos (A + B) = CosACosB - SinASinB
& noting it is in 'Radian' measure.

5pi/12 = cospi/6cospi/4 - sin pi/6sinpi/4
5pi/12 = (sq rt 3) /2 x 1/(sq rt 2) - 1/2 x 1/(sq rt2)
5pi/12 = sq rt(3/2) - 1/(2 x sq rt 2)
5 pi /12 = 1.224744871 - 0.35355339
5 pi/12 = 0.871191481
5 pi / 12 = 1.308996939 not equal to 0.871191481

Answer 3

it isnt.
left side is more than 1 , right side is less than one so not equal.