The teacher will be very surprised to see your correct solution 😉
/ 2 \ \d - 2*d + 1/*y = cos(3*x)
-1 - w + d+2+2*d+1y = 0
-1 - w + y*(1 + d^2 - 2*d) = 0
w = 1 / ((-w + y*(1 + d^2 - 2*d))/w)
/ cos(3*x) \ / cos(3*x) \ y1 = I*im|------------| + re|------------| | 2 | | 2 | \1 + d - 2*d/ \1 + d - 2*d/
y1 = re(cos(3*x)/(d^2 - 2*d + 1)) + i*im(cos(3*x)/(d^2 - 2*d + 1))
sum
/ cos(3*x) \ / cos(3*x) \ I*im|------------| + re|------------| | 2 | | 2 | \1 + d - 2*d/ \1 + d - 2*d/
=
/ cos(3*x) \ / cos(3*x) \ I*im|------------| + re|------------| | 2 | | 2 | \1 + d - 2*d/ \1 + d - 2*d/
product
/ cos(3*x) \ / cos(3*x) \ I*im|------------| + re|------------| | 2 | | 2 | \1 + d - 2*d/ \1 + d - 2*d/
=
/ cos(3*x) \ / cos(3*x) \ I*im|------------| + re|------------| | 2 | | 2 | \1 + d - 2*d/ \1 + d - 2*d/
i*im(cos(3*x)/(1 + d^2 - 2*d)) + re(cos(3*x)/(1 + d^2 - 2*d))