Cos⁡ (_^4) 13x-sin⁡ (_^4) 13x=cos⁡7x

Cos⁡ (_^4) 13x-sin⁡ (_^4) 13x=cos⁡7x

(Cos⁡ (_^2) 13x-sin⁡ (_^2) 13x) (Cos⁡ (_^2) 13x+sin⁡ (_^2) 13x) = cos⁡7x

(Cos⁡ (_^2) 13x-sin⁡ (_^2) 13x) = cos⁡7x

cos⁡14x=cos⁡7x

cos⁡14x-cos⁡7x=0

cos (α) - cos (β) = - 2sin (½ (α+β)) sin (½ (α-β))

-2sin (½ (14x+7x)) sin (½ (14x-7x)) = 0

1) sin (21x/2) = 0 2) sin ((7x) / 2) = 0

21x/2 = πn, n∈Z 7x/2=πn, n∈Z

x=2πn/21, n∈Z x=2πn/7, n∈Z