{Calculation of Impedance}
\[ Z = \sqrt{(X_L - X_C)^2 + R^2} \] Given that \( X_L = X_C \), the formula simplifies to \[ Z = \sqrt{(R - R)^2 + R^2} = \sqrt{0 + R^2} = R \] Therefore, the impedance is \( R \).
A particle is moving in a straight line. The variation of position $ x $ as a function of time $ t $ is given as:
$ x = t^3 - 6t^2 + 20t + 15 $.
The velocity of the body when its acceleration becomes zero is: