>>94
f(-1) = 1-a+b = 4

f'(x) = 3x^2 +a
f'(-1) = 3 + a = 0
a = -3
b = 0
となり
f'(x) = 3x^2 -3 = 3(x-1)(x+1)

x=1で極小となり、f(1) = -2