Help with Game Theory question

Okay kind of a basic one here, just going through the Gibbons book question 1.5,

I'm trying to calculate the both payoffs in the monopoly game

I have my profit function

And my quantities in monopoly are qm - after maximising profits

since in monopoly

my profits are (a-c)^2 / 4

which is wrong and should be divided by 8 - not sure where I have gone wrong...

Re: Help with Game Theory question

I cant tell where you went wrong because you jumped from the expression for qi and qj to the final profit. Here's the solution.