$$$t \left(t - 1\right)$$$の導関数

積の微分法 $$$\frac{d}{dt} \left(f{\left(t \right)} g{\left(t \right)}\right) = \frac{d}{dt} \left(f{\left(t \right)}\right) g{\left(t \right)} + f{\left(t \right)} \frac{d}{dt} \left(g{\left(t \right)}\right)$$$ を $$$f{\left(t \right)} = t$$$ と $$$g{\left(t \right)} = t - 1$$$ に適用する:

$${\color{red}\left(\frac{d}{dt} \left(t \left(t - 1\right)\right)\right)} = {\color{red}\left(\frac{d}{dt} \left(t\right) \left(t - 1\right) + t \frac{d}{dt} \left(t - 1\right)\right)}$$

$$$n = 1$$$ を用いて冪法則 $$$\frac{d}{dt} \left(t^{n}\right) = n t^{n - 1}$$$ を適用すると、すなわち $$$\frac{d}{dt} \left(t\right) = 1$$$:

$$t \frac{d}{dt} \left(t - 1\right) + \left(t - 1\right) {\color{red}\left(\frac{d}{dt} \left(t\right)\right)} = t \frac{d}{dt} \left(t - 1\right) + \left(t - 1\right) {\color{red}\left(1\right)}$$

和/差の導関数は、導関数の和/差である:

$$t {\color{red}\left(\frac{d}{dt} \left(t - 1\right)\right)} + t - 1 = t {\color{red}\left(\frac{d}{dt} \left(t\right) - \frac{d}{dt} \left(1\right)\right)} + t - 1$$

定数の導数は$$$0$$$です:

$$t \left(- {\color{red}\left(\frac{d}{dt} \left(1\right)\right)} + \frac{d}{dt} \left(t\right)\right) + t - 1 = t \left(- {\color{red}\left(0\right)} + \frac{d}{dt} \left(t\right)\right) + t - 1$$

$$$n = 1$$$ を用いて冪法則 $$$\frac{d}{dt} \left(t^{n}\right) = n t^{n - 1}$$$ を適用すると、すなわち $$$\frac{d}{dt} \left(t\right) = 1$$$:

$$t {\color{red}\left(\frac{d}{dt} \left(t\right)\right)} + t - 1 = t {\color{red}\left(1\right)} + t - 1$$

したがって、$$$\frac{d}{dt} \left(t \left(t - 1\right)\right) = 2 t - 1$$$。