1/3*[1/x-1/(x+3)+1/(x+3)-1/(x+6)+……+1/(x+33)-1/(x+36)]=3/40
1/x-1/(x+36)=9/40
36/x(x+36)=9/40
x^2+36x=160
(x-4)(x+40)=0
x=4,x=-40