Twierdzenie Pitagorasa
a^2+b^2=c^2
a = 2
b = 4
c = ?
c^2=2^2+4^2
c^2=16+4
c=\sqrt{20}=\sqrt{4\cdot 5}
c=2\sqrt5
sin\alpha = \frac{a}{c}
cos\alpha = \frac{b}{c}
sin\alpha\cdot cos \alpha =\frac{a}{c}\cdot \frac{b}{c}=\frac{2}{2\sqrt5}\cdot \frac{4}{2\sqrt5}=\frac{8}{4 \cdot 5}=\frac{8}{20}= 0,4