integral of cos(ax)cos(bx)

解

\int cos (a x) cos (b x) d x

\frac{1}{2} (\frac{1}{a + b} sin (a x + b x) + \frac{1}{a - b} sin (a x - b x)) + C

解答ステップ

\int cos (a x) cos (b x) d x

三角関数の公式を使用して書き換える

= \int \frac{cos ( a x + b x ) + cos ( a x - b x )}{2} d x

定数を除く:

\int a \cdot f (x) d x = a \cdot \int f (x) d x

= \frac{1}{2} \cdot \int cos (a x + b x) + cos (a x - b x) d x

総和規則を適用する:

\int f (x) \pm g (x) d x = \int f (x) d x \pm \int g (x) d x

= \frac{1}{2} (\int cos (a x + b x) d x + \int cos (a x - b x) d x)

\int cos (a x + b x) d x = \frac{1}{a + b} sin (a x + b x)

\int cos (a x - b x) d x = \frac{1}{a - b} sin (a x - b x)

= \frac{1}{2} (\frac{1}{a + b} sin (a x + b x) + \frac{1}{a - b} sin (a x - b x))

定数を解答に追加する

= \frac{1}{2} (\frac{1}{a + b} sin (a x + b x) + \frac{1}{a - b} sin (a x - b x)) + C