Question

Tính C=2.3+3.4+4.5+……+49.50

Answer 1

REFER

Đặt A=1.2+2.3+3.4+4.5+...+49.50

3A=1.2.3+2.3.3+3.4.3+4.5.3+...+49.50.3

3A=1.2.(3-0)+2.3.(4-1)+3.4.(5-2)+4.5.(6-3)+...+49.50.(51-48)

3A=1.2.3-0.1.2+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+...+49.50.51-48.49.50

3A=(1.2.3+2.3.4+3.4.5+4.5.6+...+49.50.51)-(0.1.2+1.2.3+2.3.4+3.4.5+...+48.49.50)

3A=49x50x51-0x1x2

3A=49 x 50 x 51

A = 49 x 50 x 17

A = 41650

Answer 2

có thêm 1.2 nx đc khum

Answer 3

Tham khảo

A=1.2+2.3+...+49.50

3A=1.2.3+2.3.3+...+49.50.3

3A=1.2.(4-1)+2.3.(5-2)+....+49.50.(51-48)

3A=1.2.4-1.2.1+2.3.5-2.3.2+...+49.50.51-49.50.48

3A=49.50.51

=>A=49.25.51

=>A=62475