8.求下列幂级数的收敛半径及其和函数:-|||-(1) sum _(n=1)^infty dfrac ({x)^n}(n(n+1)) ;-|||-(2) sum _(n=1)^infty dfrac ({x)^n}(n(n+1)(n+2)) :-|||-(3) sum _(n=2)^infty dfrac ({(n-1))^2}(n+1)(x)^n (提示: ((n-1))^2=([ (n+1)-2] )^2=((n+1))^2-4(n+1)+4)