Write cos(5πt) cos(2πt) as a product of two trigonometric functions of different frequencies.

a) 0.5[cos(7πt) + cos(3πt)]
b) 0.5[cos(3πt) + cos(7πt)]
c) 0.5[cos(5πt) + cos(2πt)]
d) 0.5[cos(2πt) + cos(5πt)]