(TAN(x) - SIN(x)) / SIN(x)3
mit TAN(x) = SIN(x) / COS(x)
= (SIN(x)/COS(x) - SIN(x))/SIN(x)3
= (1 - COS(x)) / (SIN(x)2·COS(x))
= (1 - COS(x)) / ((1 - COS(x)2)·COS(x))
= (1 - COS(x)) / ((1 + COS(x))·(1 - COS(x))·COS(x))
= 1 / ((1 + COS(x))·COS(x))
lim x --> 0
= 1/2