Integral dari $$$e^{\frac{x^{2}}{2}}$$$

Temukan $$$\int e^{\frac{x^{2}}{2}}\, dx$$$.

Misalkan $$$u=\frac{\sqrt{2} x}{2}$$$.

Kemudian $$$du=\left(\frac{\sqrt{2} x}{2}\right)^{\prime }dx = \frac{\sqrt{2}}{2} dx$$$ (langkah-langkah dapat dilihat di »), dan kita memperoleh $$$dx = \sqrt{2} du$$$.

Integralnya menjadi

$${\color{red}{\int{e^{\frac{x^{2}}{2}} d x}}} = {\color{red}{\int{\sqrt{2} e^{u^{2}} d u}}}$$

Terapkan aturan pengali konstanta $$$\int c f{\left(u \right)}\, du = c \int f{\left(u \right)}\, du$$$ dengan $$$c=\sqrt{2}$$$ dan $$$f{\left(u \right)} = e^{u^{2}}$$$:

$${\color{red}{\int{\sqrt{2} e^{u^{2}} d u}}} = {\color{red}{\sqrt{2} \int{e^{u^{2}} d u}}}$$

Integral ini (Fungsi Galat Imajiner) tidak memiliki bentuk tertutup:

$$\sqrt{2} {\color{red}{\int{e^{u^{2}} d u}}} = \sqrt{2} {\color{red}{\left(\frac{\sqrt{\pi} \operatorname{erfi}{\left(u \right)}}{2}\right)}}$$

Ingat bahwa $$$u=\frac{\sqrt{2} x}{2}$$$:

$$\frac{\sqrt{2} \sqrt{\pi} \operatorname{erfi}{\left({\color{red}{u}} \right)}}{2} = \frac{\sqrt{2} \sqrt{\pi} \operatorname{erfi}{\left({\color{red}{\left(\frac{\sqrt{2} x}{2}\right)}} \right)}}{2}$$

Oleh karena itu,

$$\int{e^{\frac{x^{2}}{2}} d x} = \frac{\sqrt{2} \sqrt{\pi} \operatorname{erfi}{\left(\frac{\sqrt{2} x}{2} \right)}}{2}$$

Tambahkan konstanta integrasi:

$$\int{e^{\frac{x^{2}}{2}} d x} = \frac{\sqrt{2} \sqrt{\pi} \operatorname{erfi}{\left(\frac{\sqrt{2} x}{2} \right)}}{2}+C$$

$$$\int e^{\frac{x^{2}}{2}}\, dx = \frac{\sqrt{2} \sqrt{\pi} \operatorname{erfi}{\left(\frac{\sqrt{2} x}{2} \right)}}{2} + C$$$A