Question

Tinh tong S = 1×2+2×3+3×4+4×5+•••••+99×100

Answer 1

 S=1*2+2*3+3*4+...+99*100

3S=3*(1*2+2*3+3*4+...+99*100)

3S=1*2*3+2*3*3+3*4*3+...+99*100*3

3S=1*2*(3-0)+2*3*(4-1)+3*4*(5-2)+...+99*100*(101-98)

3S=1*2*3-1*2*0+2*3*4-2*3*1+3*4*5-3*4*2+...+99*100*101-99*100*98

3S=(1*2*3-2*3*1)+(2*3*4-3*4*2)+...+(98*99*100-99*100*98)+99*100*101

3S=0+0+...+0+999900

3S=999900

 S=999900/3

 S=333300

Answer 2

3S = 1.2.3 + 2.3.3 + 3.4.3 +...+99.100.3

=1.2.3 + 2.3.(4-1)+3.4(5-2)+...+99.100(101-98)

=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+99.100.101-98.99.100

= 99.100.101

=999900

Answer 3

Nga Nguyễn ơi, bạn chưa chia cho 3 rồi