Find the exact value of the expression cos 20 degree cos 10

Find the exact value of the expression. cos 20 degree cos 10 degree - sin 20 degree sin 10 degree cos 20 degree cos 10 degree - sin 20 degree sin 10 degree = (Simplify your answer, including any radicals. Use integers or fractions for any numbers in the expression.)

Solution

cosAcosB-sinAsinB=cos(A+B)

cos20^ocos10^o-sin20^osin10^o=cos(20^o+10^o)

cos20^ocos10^o-sin20^osin10^o=cos(30^o)

cos20^ocos10^o-sin20^osin10^o=(3)_/2