Trigonometry/e to the i Pi

Take the formula
 * $$e^{xi}=\cos(x)+i\sin(x)$$.

If $$x=\pi$$ radians, $$\cos(x)=-1$$ and $$\sin(x)=0$$ so the formula becomes
 * $$e^{\pi i}=-1$$

or
 * $$e^{\pi i}+1=0$$

a formula relating five key numbers in mathematics.