Đáp án:
a,$C= 1.2+2.3+3.4+4.5+...+99.100$
$3C=1.2.3+2.3.(4-1)+3.4.(5-2)+4.5.(6-3)+...+99.100.(101-98)$
$3C=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+...+99.100.101-98.99.100$
$3C=99.100.101$
$C=33.100.101=333300$
b,$D = 2^2+4^2+6^2+...+98^2$
$D=2².(1+2²+3²+...+49²)$
$D=2².[1(0+1)+2.(1+1)+3.(2+1)+...+49.(48+1)]$
$D=2².[(0+1+2+3+...+49)+(1.2+2.3+...+48.49)]$
$D=4.[1225+39200]$
$D=161700$