455. The amount of electricity flowing through a conductor, starting from the moment of time $t=0$, is given by the formula $Q=$ $=3 t^{2}-3 t+4$. Find the current strength at the end of the 6th second.
Solution. The current is the derivative of the quantity of electricity with respect to time: therefore, we need to find the derivative of the function $Q=3 t^{2}-3 t+4$ and calculate its value at $t=6 \mathrm{c}$. We have $I=Q^{\prime}=$ $=6 t-3$, from which at $t=6$ we get $I=33$ (A).