هنا

[hossam hassan]

كيف أستطيع ضرب cos x فى cos Kx حيث K ثابت ف المسألة

[mishwar]

There are many ways, It depends on why you need to multiply them.

for example you can use this law:
cosa * cosb = 1/2[cos(a-b) + cos(a+b)]

let a = kx and b = x

and do the rest