Question

[Lưu Gia Hưng]

1*1+2*2+3*3+4*4+5*5+6*6+......+20*20

Answer 1

[Nguyễn Thị Thương Hoài]

A = 1x1 + 2x2 + 3x3 +...+ 20x20

A = 1x1 + (1 + 1) x 2 + (1 + 2) x 3 + .... + (1 + 19) x 20

A = 1 + 2 + 1x2 + 3 + 2x3 +...+ 20 x 19 x 20

A = (1 + 2+...+20) + (1x2 + 2x3 +...+19x20)

C = 1 + 2 +...+ 20; B = 1x2 + 2x3 +...+19x20

C = (20 + 1)x20: 2

C =  210

B = \(\dfrac{1}{3}\) x(1x2x3 + 2x3x3 +...+19x20x3)

B = \(\dfrac{1}{3}\)x(1x2x3 + 2x3x(4-1) +...+ 19x20x(21-18)

B = \(\dfrac{1}{3}\)x(1x2x3 +2x3x4 - 1x2x3+...+ 19x20x21 - 18x19x20)

B = \(\dfrac{1}{3}\) x 19x20x21

B = 2660

A = 210 + 2660 

A = 2870

Answer 2

[Lê Ngọc Bảo An]

Khó qué t lớp 3 còn chưa hiểu rõ 🙈🫥😙