limx→1xn−1−n(x−1)(x−1)2=
limx→1(x−1)(xn−1+xn−2+...+x+1−n)(x−1)2=
limx→1xn−1+xn−2+...+x+1−n(x−1)=
limx→1xn−1−1+xn−2−1+..+x−1(x−1)=
limx→1xn−1−1x−1+limx→1xn−2−1x−1+...+limx→1x−1x−1=
limx→1(xn−2+xn−3+....+x+1)+limx→1(xn−3+xn−4+....+x+1)+...+limx→1(1)=
(n−1)+(n−2)+...+2+1=n(n−1)2
Làm đúng nhớ vote nhé, ủng hộ tinh thần