$$$\left[\begin{array}{ccc}\cos{\left(x \right)} & \sin{\left(x \right)} & \sin{\left(2 x \right)}\\- \sin{\left(x \right)} & \cos{\left(x \right)} & 2 \cos{\left(2 x \right)}\\- \cos{\left(x \right)} & - \sin{\left(x \right)} & - 4 \sin{\left(2 x \right)}\end{array}\right]$$$の行列式

$$$\left|\begin{array}{ccc}\cos{\left(x \right)} & \sin{\left(x \right)} & \sin{\left(2 x \right)}\\- \sin{\left(x \right)} & \cos{\left(x \right)} & 2 \cos{\left(2 x \right)}\\- \cos{\left(x \right)} & - \sin{\left(x \right)} & - 4 \sin{\left(2 x \right)}\end{array}\right|$$$を計算せよ。

列$$$2$$$から$$$\frac{\sin{\left(x \right)}}{\cos{\left(x \right)}}$$$倍の列$$$1$$$を引く: $$$C_{2} = C_{2} - \frac{\sin{\left(x \right)}}{\cos{\left(x \right)}} C_{1}$$$。

$$$\left|\begin{array}{ccc}\cos{\left(x \right)} & \sin{\left(x \right)} & \sin{\left(2 x \right)}\\- \sin{\left(x \right)} & \cos{\left(x \right)} & 2 \cos{\left(2 x \right)}\\- \cos{\left(x \right)} & - \sin{\left(x \right)} & - 4 \sin{\left(2 x \right)}\end{array}\right| = \left|\begin{array}{ccc}\cos{\left(x \right)} & 0 & \sin{\left(2 x \right)}\\- \sin{\left(x \right)} & \frac{1}{\cos{\left(x \right)}} & 2 \cos{\left(2 x \right)}\\- \cos{\left(x \right)} & 0 & - 4 \sin{\left(2 x \right)}\end{array}\right|$$$

列$$$3$$$から$$$\frac{\sin{\left(2 x \right)}}{\cos{\left(x \right)}}$$$倍の列$$$1$$$を引く: $$$C_{3} = C_{3} - \frac{\sin{\left(2 x \right)}}{\cos{\left(x \right)}} C_{1}$$$。

$$$\left|\begin{array}{ccc}\cos{\left(x \right)} & 0 & \sin{\left(2 x \right)}\\- \sin{\left(x \right)} & \frac{1}{\cos{\left(x \right)}} & 2 \cos{\left(2 x \right)}\\- \cos{\left(x \right)} & 0 & - 4 \sin{\left(2 x \right)}\end{array}\right| = \left|\begin{array}{ccc}\cos{\left(x \right)} & 0 & 0\\- \sin{\left(x \right)} & \frac{1}{\cos{\left(x \right)}} & 2 \cos^{2}{\left(x \right)}\\- \cos{\left(x \right)} & 0 & - 3 \sin{\left(2 x \right)}\end{array}\right|$$$

第$$$1$$$行に沿って展開:

$$$\left|\begin{array}{ccc}\cos{\left(x \right)} & 0 & 0\\- \sin{\left(x \right)} & \frac{1}{\cos{\left(x \right)}} & 2 \cos^{2}{\left(x \right)}\\- \cos{\left(x \right)} & 0 & - 3 \sin{\left(2 x \right)}\end{array}\right| = \left(\cos{\left(x \right)}\right) \left(-1\right)^{1 + 1} \left|\begin{array}{cc}\frac{1}{\cos{\left(x \right)}} & 2 \cos^{2}{\left(x \right)}\\0 & - 3 \sin{\left(2 x \right)}\end{array}\right| + \left(0\right) \left(-1\right)^{1 + 2} \left|\begin{array}{cc}- \sin{\left(x \right)} & 2 \cos^{2}{\left(x \right)}\\- \cos{\left(x \right)} & - 3 \sin{\left(2 x \right)}\end{array}\right| + \left(0\right) \left(-1\right)^{1 + 3} \left|\begin{array}{cc}- \sin{\left(x \right)} & \frac{1}{\cos{\left(x \right)}}\\- \cos{\left(x \right)} & 0\end{array}\right| = \cos{\left(x \right)} \left|\begin{array}{cc}\frac{1}{\cos{\left(x \right)}} & 2 \cos^{2}{\left(x \right)}\\0 & - 3 \sin{\left(2 x \right)}\end{array}\right|$$$

2x2行列の行列式は$$$\left|\begin{array}{cc}a & b\\c & d\end{array}\right| = a d - b c$$$です。

$$$\left|\begin{array}{cc}\frac{1}{\cos{\left(x \right)}} & 2 \cos^{2}{\left(x \right)}\\0 & - 3 \sin{\left(2 x \right)}\end{array}\right| = \left(\frac{1}{\cos{\left(x \right)}}\right)\cdot \left(- 3 \sin{\left(2 x \right)}\right) - \left(2 \cos^{2}{\left(x \right)}\right)\cdot \left(0\right) = - 6 \sin{\left(x \right)}$$$

最後に、$$$\left(\cos{\left(x \right)}\right)\cdot \left(- 6 \sin{\left(x \right)}\right) = - 3 \sin{\left(2 x \right)}$$$。

$$$\left|\begin{array}{ccc}\cos{\left(x \right)} & \sin{\left(x \right)} & \sin{\left(2 x \right)}\\- \sin{\left(x \right)} & \cos{\left(x \right)} & 2 \cos{\left(2 x \right)}\\- \cos{\left(x \right)} & - \sin{\left(x \right)} & - 4 \sin{\left(2 x \right)}\end{array}\right| = - 3 \sin{\left(2 x \right)}$$$A