given data, cos(A - B) = cosAcosB + sinAsinB
let, A=60' and B=30' ( here the ' sign bears degree)
L.H.S = cos(A - B)
=cos (60'-30) ( using value of A and B )
=cos30'
= sqrt3/2
R.H.S= cosAcosB + sinAsinB
=cos60' cos30' + sin60' sin30'
= 1/2* sqrt3/2+ sqrt3/2*1/2
= sqrt3/4 + sqrt3/4
=2 sqrt3/4
= sqrt3/2
so L.H.S =R.H.S or cos(A - B) = cosAcosB + sinAsinB