The question presumably means that b≠0.
a=1/2 and b = 1 and c=-(1/2) , i.e. f(x) for x ≥ 1 = (1/2)x2 + x - (1/2), then f'(x) = x + 1
f(1)=1 and
and lim (as x approaches 1 from above) f'(x) = 2
and lim (as x approaches 1 from below) f'(x) = 2
and f'(1) = 2