ex = 1 + x + x2/2! + x3/3! + x4/4! + x5/5! + x6/6!...
cos(x) = 1 - x2/2! + x4/4! - x6/6!...
sin(x) x - x3/3! + x5/5!...
cos(x) = 1 - x2/2! + x4/4! - x6/6!...
sin(x) x - x3/3! + x5/5!...
So...
ex*i = 1 + (x*i) + (x*i)2/2! + (x*i)3/3! + (x*i)4/4! + (x*i)5/5! + (x*i)6/6!...
= 1 + (x*i) + (x2*i2)/2! + (x3*i3)/3! + (x4*i4)/4! + (x5*i5)/5! + (x6*i6)/6!...
= 1 + x*i - x2/2! - (x3*i)/3! + x4/4! + (x5*i)/5! - x6/6!...
= (1 - x2/2! + x4/4! - x6/6!...) + i*(x - x3/3! + x5/5!...)
= cos(x) + i*sin(x)
= 1 + (x*i) + (x2*i2)/2! + (x3*i3)/3! + (x4*i4)/4! + (x5*i5)/5! + (x6*i6)/6!...
= 1 + x*i - x2/2! - (x3*i)/3! + x4/4! + (x5*i)/5! - x6/6!...
= (1 - x2/2! + x4/4! - x6/6!...) + i*(x - x3/3! + x5/5!...)
= cos(x) + i*sin(x)
Thus...
epi*i + 1 = cos(pi) + i*sin(pi) + 1
= -1 + 0 + 1 = 0
= -1 + 0 + 1 = 0
Prety spiffy, huh?