CSC: $U_n=U_1+(n-1)d$Ta co : $S=U_1^3+U^3_2+...+U^3_{100}=U_1^3+(U_1+d)^3+(U_1+2d)^3+...+(U_1+99d)^3$
$=100U_1^3+3U_1^2d(1+2+3+...+99)+3U_1d^2(1^2+2^2+3^2+..+99^2)+d^3(1^3+2^2+..+99^3)$
$=100U_1^3+3U_1^2d\frac{99(1+99)}{2}+3U_1d^2\frac{99(99+1)(2.99+1)}{6}+d^3\frac{99^2(99+1)^2}{4}$
Thay so, tu tinh tiep nha:)