For an AP, use the formula \( S_n = \frac{n}{2}(a + l) \) to find the sum.
2300
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: