------------------------------
X(13-x)/(x+6)>0
x=0 x=13 x=-6
+ _ + _
----------------(-6)--------------(0)---------------(13)----------------
x∈(-∞;-6) U (0;13)
Q^2=b5/b3=2,25
q=1,5
b4=b3*q=4*1,5=6
Cos3xcos5x=1/2(cos(3x-5x)+cos(3x+5x)=1/2(cos2x+cos8x)