Integral dari $$$\cos{\left(3 x^{2} \right)}$$$

Temukan $$$\int \cos{\left(3 x^{2} \right)}\, dx$$$.

Misalkan $$$u=\sqrt{3} x$$$.

Kemudian $$$du=\left(\sqrt{3} x\right)^{\prime }dx = \sqrt{3} dx$$$ (langkah-langkah dapat dilihat di »), dan kita memperoleh $$$dx = \frac{\sqrt{3} du}{3}$$$.

Jadi,

$${\color{red}{\int{\cos{\left(3 x^{2} \right)} d x}}} = {\color{red}{\int{\frac{\sqrt{3} \cos{\left(u^{2} \right)}}{3} d u}}}$$

Terapkan aturan pengali konstanta $$$\int c f{\left(u \right)}\, du = c \int f{\left(u \right)}\, du$$$ dengan $$$c=\frac{\sqrt{3}}{3}$$$ dan $$$f{\left(u \right)} = \cos{\left(u^{2} \right)}$$$:

$${\color{red}{\int{\frac{\sqrt{3} \cos{\left(u^{2} \right)}}{3} d u}}} = {\color{red}{\left(\frac{\sqrt{3} \int{\cos{\left(u^{2} \right)} d u}}{3}\right)}}$$

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

$$\frac{\sqrt{3} {\color{red}{\int{\cos{\left(u^{2} \right)} d u}}}}{3} = \frac{\sqrt{3} {\color{red}{\left(\frac{\sqrt{2} \sqrt{\pi} C\left(\frac{\sqrt{2} u}{\sqrt{\pi}}\right)}{2}\right)}}}{3}$$

Ingat bahwa $$$u=\sqrt{3} x$$$:

$$\frac{\sqrt{6} \sqrt{\pi} C\left(\frac{\sqrt{2} {\color{red}{u}}}{\sqrt{\pi}}\right)}{6} = \frac{\sqrt{6} \sqrt{\pi} C\left(\frac{\sqrt{2} {\color{red}{\sqrt{3} x}}}{\sqrt{\pi}}\right)}{6}$$

Oleh karena itu,

$$\int{\cos{\left(3 x^{2} \right)} d x} = \frac{\sqrt{6} \sqrt{\pi} C\left(\frac{\sqrt{6} x}{\sqrt{\pi}}\right)}{6}$$

Tambahkan konstanta integrasi:

$$\int{\cos{\left(3 x^{2} \right)} d x} = \frac{\sqrt{6} \sqrt{\pi} C\left(\frac{\sqrt{6} x}{\sqrt{\pi}}\right)}{6}+C$$

$$$\int \cos{\left(3 x^{2} \right)}\, dx = \frac{\sqrt{6} \sqrt{\pi} C\left(\frac{\sqrt{6} x}{\sqrt{\pi}}\right)}{6} + C$$$A