Part circle(radius is "r" ) is made from a string.string's lenght is "L".so the part

circle's circumference is also "L".If part circle's area is "A",Find the angle of the part

circle when Area is maximum

,here what I did but never got the answer.what is wrong in this?

setting to zero for the maximum value

now what? answer has to be r(L-2r)/2(as books says).