Question

C=1+3+6+10+15+...+1225

Answer 1

2xC=2+6+12+20+...+2450

2xC=1x2+2x3+3x4+4x5+...+49x50

6xC=1x2x3+2x3x3+3x4x3+...+49x50x3

6xC=1x2x3+2x3x(4-1)+3x4x(5-2)+...+49x50x(51-48)

6xC=1x2x3+2x3x4-1x2x3+....+49x50x51-48x49x50

6xC=49x50x51

6xC=124950

C=20825

k nhé đúng 100%

Answer 2

2C = 2+6+12+20+30+...+2450

2C = 1.2+2.3+3.4+4.5+5.6+...+49.50

3C = 1.2.3+2.3.3+3.4.3+4.5.3+5.6.3+...+49.50.3

3C = 1.2.3+2.3.(4-1)+3.4.(5-2)+4.5.(6-3)+5.6.(7-4)+...+49.50.(51-48)

3C = 1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+5.6.7-4.5.6+5.6.7-4.5.6+....+49.50.51-48.49.50

=>C=49.50.51 : 3 =>C=4650