Neidio i'r cynnwys

# Fformiwla Euler

Mae fformiwla Euler yn nodi fod:

${\displaystyle e^{i\theta }=\cos(\theta )+\sin(\theta )i\,}$

ble mae ${\displaystyle i}$ yn rhif dychmygol sydd yn sgwario i roi ${\displaystyle -1}$.

Daw'r enw "fformiwla Euler" ar ôl y mathemategydd Leonhard Euler.

## Prawf

Mae hyn yn deillio o ehangiadau Cyfres Taylor sy'n nodi fod:

${\displaystyle e^{x}=1+x+{\frac {x^{2}}{2!}}+{\frac {x^{3}}{3!}}+{\frac {x^{4}}{4!}}+{\frac {x^{5}}{5!}}+{\frac {x^{6}}{6!}}+{\frac {x^{7}}{7!}}+...+{\frac {x^{p}}{p!}}}$
${\displaystyle \cos {\theta }=1-{\frac {\theta ^{2}}{2!}}+{\frac {\theta ^{4}}{4!}}-{\frac {\theta ^{6}}{6!}}+...+{\frac {(-1)^{p}\theta ^{2p}}{(2p)!}}+...}$
${\displaystyle \sin {\theta }=\theta -{\frac {\theta ^{3}}{3!}}+{\frac {\theta ^{5}}{5!}}-{\frac {\theta ^{7}}{7!}}+...+{\frac {(-1)^{p}\theta ^{2p+1}}{(2p+1)!}}+...}$

Wedyn o gyfnewid ${\displaystyle x=i\theta }$ yn ehangiad Cyfres Taylor ar gyfer ${\displaystyle e^{x}}$ rydym yn cael:

${\displaystyle e^{i\theta }=1+(i\theta )+{\frac {(i\theta )^{2}}{2!}}+{\frac {(i\theta )^{3}}{3!}}+{\frac {(i\theta )^{4}}{4!}}+{\frac {(i\theta )^{5}}{5!}}+{\frac {(i\theta )^{6}}{6!}}+{\frac {(i\theta )^{7}}{7!}}+...}$
${\displaystyle =1+(i\theta )+{\frac {i^{2}\theta ^{2}}{2!}}+{\frac {ii^{2}\theta ^{3}}{3!}}+{\frac {i^{2}i^{2}\theta ^{4}}{4!}}+{\frac {ii^{2}i^{2}\theta ^{5}}{5!}}+{\frac {i^{2}i^{2}i^{2}\theta ^{6}}{6!}}+{\frac {ii^{2}i^{2}i^{2}\theta ^{7}}{7!}}+...}$
${\displaystyle =1+(i\theta )-{\frac {\theta ^{2}}{2!}}-i{\frac {\theta ^{3}}{3!}}+{\frac {\theta ^{4}}{4!}}+i{\frac {\theta ^{5}}{5!}}-{\frac {\theta ^{6}}{6!}}-i{\frac {\theta ^{7}}{7!}}+...}$
${\displaystyle =\{1-{\frac {\theta ^{2}}{2!}}+{\frac {\theta ^{4}}{4!}}-{\frac {\theta ^{6}}{6!}}+...\}+i\{\theta -{\frac {\theta ^{3}}{3!}}+{\frac {\theta ^{5}}{5!}}-i{\frac {\theta ^{7}}{7!}}+...\}}$
${\displaystyle =\cos {(\theta )}+\sin {(\theta )i}\,}$