4.利用定积分求极限:-|||-(1) lim _(narrow infty )dfrac (1)({n)^4}(1+(2)^3+... +(n)^3);-|||-(2) lim _(narrow infty )n[ dfrac (1)({(n+1))^2}+dfrac (1)({(n+2))^2}+... +dfrac (1)({(n+n))^2}] ;-|||-(3) lim _(narrow infty )n(dfrac (1)({n)^2+1}+dfrac (1)({n)^2+(2)^2}+... +dfrac (1)(2{n)^2});-|||-(4) lim _(narrow infty )dfrac (1)(n)(sin dfrac (pi )(n)+sin dfrac (2pi )(n)+... +sin dfrac (n-1)(n)pi )