Beauty of Mathematics

From $x^2 + y^2 = 1$ , we assum $x = \cos(a)$ and $y = \sin(a)$

Now, $20x^3 - 15x = 3$

$4x^3 - 3x = \frac{3}{5}$

$4\cos^3(a) - 3\cos(a) = \frac{3}{5}$

$\cos(3a) = \frac{3}{5}$

Then, $\sin(3a) = \frac{4}{5}, \frac{-4}{5}$

$3\sin(a) - 4\sin^3(a) = \frac{4}{5}$

$3y - 4y^3 = \frac{4}{5}$

$15y - 20y^3 = 4$

$|20y^3 - 15y| = 4$