cos7 cos9 0Solutioncos7 cos9 0 Use the trig formula cosA

cos(7) cos(9) = 0

Use the trig. formula : cosA - cosB = 2sin{(A+B)/2}sin{(B-A)/2}

cos(7) cos(9) = 2sin( 16)/2 sin( -) =0

Sol-I : sin(16) =0

= 2pi*k/16 , (pi +2pi*k)/16

= pi*k/8 , (pi +2pi*k)/16

Sol -II :sin(-) =0

= -2pi*k , -2pi*k -pi

Solution -I and II give general solution of the equation.