Using the eqn: f(qi,Q-i)= P(qi+Q-i) - c + qiP'Q(qi+Q-i)=0

and the implicit fn rule: dQ-i/dqi = -df/dqi / df/dQ-i

= 2P'(Q) + qiP''(Q)/ P'(Q)+qiP''(Q)

why is it 2P'(Q) on the numerator and only P'(Q) on the denominator?? Any explanation would be greatly appreciated