81a-a^3=a (81-a^2)=a (9-a)(9+a)
sin(x) - sin(4x) = sin(3x) - sin(2x)
-2cos(5x/2)sin(3x/2) = 2cos(5x/2)sin(x/2)
-cos(5x/2)sin(3x/2) = cos(5x/2)sin(x/2)
-sin(3x/2) = sin(x/2)
-(sin(3x/2) + sin(x/2)) = 0
sin(3x/2) + sin(x/2) = 0
2sin(x)cos(x/2) = 0
<h2>1. sin(x) = 0</h2>
x = (-1)ⁿarcsin(0) + πn, n ∈ Z
<u>x = πn, n ∈ Z</u>
<h2>2. cos(x/2) = 0</h2>
x/2 = arccos0 + πn, n ∈ Z
<u>x = π + 2πn, n ∈ Z</u>