(a) T.C = 60 - 12x - 2x2
Average cost = \(\frac {T.C.}x = \frac {60 + 2x - 2x^2}x\)
(b) \(f(x) = 60 - 12x + 2x^2\)
\(f'(x) = -12 + 4x\)
\(f'(x) = 0 \) gives -12 + 4x = 0
⇒ \(x = \frac {12}{+4} = +3\)
\(f''(x) = + 4 > 0\)
Hence, x = 3 is point of minima.
Hence, at x = 3, the function is minimum.