ta có:A=1(2+1)+2(3+1)+3(4+1)+.....+99(100+1)A=1.2+1+2.3+2+3.4+3+....+99.100+99A=(1.2+2.3+3.4+...+99.100)+(1+2+3+.+99)=333300+4950=338250
ta có
$:A=1(2+1)+2(3+1)+3(4+1)+.....+99(100+1)
$A=1.2+1+2.3+2+3.4+3+....+99.100+99A=(1.2+2.3+3.4+...+99.100)+(1+2+3+.+99)=333300+4950=338250