find cos 15 degrees by using 15degrees=60deg-45deg?

sin 60 = SR (3/2) .................. cos 60 = 1/2

sin 45 = cos 45 = SR (2) / 2

With the equations from above you can finish the problem.

cos(15°) = cos(60°-45°)
= cos(60°)cos(45°) + sin(60°)sin(45°)
= (½)(√2/2) + (√3/2)(√2/2)
= (√2/2)(½+√3/2)
= (√2/2)(1+√3)/2
= (√2+√6)/4

hkyklyklhy

cos15=cos(60 -45)
cos60cos45+sin60sin45
1/2(1/sqr2)+sqr3/2(1/sqr2)
1/(2sqr2)+sqr3/2sqr2)=(1+sqr3)/2sqr2

Use cos(A-B) = cosAcosB + sinAsinB
with A=60° and B=45°.

It would help if you were familiar with the 30°-60° right triangle and the 45°-45° right triangle.