Integral dari $$$\sin{\left(\pi x^{2} \right)}$$$

Temukan $$$\int \sin{\left(\pi x^{2} \right)}\, dx$$$.

Misalkan $$$u=\sqrt{\pi} x$$$.

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

Jadi,

$${\color{red}{\int{\sin{\left(\pi x^{2} \right)} d x}}} = {\color{red}{\int{\frac{\sin{\left(u^{2} \right)}}{\sqrt{\pi}} d u}}}$$

Terapkan aturan pengali konstanta $$$\int c f{\left(u \right)}\, du = c \int f{\left(u \right)}\, du$$$ dengan $$$c=\frac{1}{\sqrt{\pi}}$$$ dan $$$f{\left(u \right)} = \sin{\left(u^{2} \right)}$$$:

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

Integral ini (Integral Fresnel Sinus) tidak memiliki bentuk tertutup:

$$\frac{{\color{red}{\int{\sin{\left(u^{2} \right)} d u}}}}{\sqrt{\pi}} = \frac{{\color{red}{\left(\frac{\sqrt{2} \sqrt{\pi} S\left(\frac{\sqrt{2} u}{\sqrt{\pi}}\right)}{2}\right)}}}{\sqrt{\pi}}$$

Ingat bahwa $$$u=\sqrt{\pi} x$$$:

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

Oleh karena itu,

$$\int{\sin{\left(\pi x^{2} \right)} d x} = \frac{\sqrt{2} S\left(\sqrt{2} x\right)}{2}$$

Tambahkan konstanta integrasi:

$$\int{\sin{\left(\pi x^{2} \right)} d x} = \frac{\sqrt{2} S\left(\sqrt{2} x\right)}{2}+C$$

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