c(q)=60q-12q^2+q^3. At level of output is average cost minimum?
The average cost AC is `AC(q)=c(q)/q=60-12q+q^2.`
It is a quadratic equation, its graph is a upward parabola since the coefficient of q^2 is positive.
The critical point is a minimum.
`AC'(q)=0` iff `q=6`
The average cost is minimum at the level q=6