Can someone check my answer for this question:

Find the directional derivative of f(x,y) = x/(x+y) at P(1,0) in the direction of Q (-1,-1).

Ok so the answer I am getting is 1/sqrt(2) but the answer at the back of my book is 1/sqrt(5). I have rechecked all my workings but still can't get the solution which matches the answer in the book. Can someone tell me if the answer I got is right...thanks

PS: I am not really sure about the procedure that is needed to be followed for this question because there isn't any example in the book which deals with directional derivatives in the direction of a particular point. Help please.