求解！Bertrand competition model with differentiated products

Consider the following Bertrand competition model with differentiated products. Let the demand curves for firms A and B be given by:
Qa=1/2-Pa+Pb
Qb=1/2-Pa+Pb


and let the firms' marginal costs be constant, given by cA and cB respectively.
a) Find the best-response function for firm A and firm B.


我算出来的是Pa=(1-2Ca+2Pb)/4,答案是Pa=(1+2Pb+Ca)/4

我算的思路是用Pa*Qa然后求关于Pa的导数，得出MR=MC.

不知道为何与答案不一致，求解答啊啊！