Question

1.2+ 2.3+ 3.4+ .....+ 98.99
cíu!!

Answer 1

Đặt A = 1.2 + 2 3 + 3.4 + ... + 98.99

3A = 1.2.3 + 2.3.3 + 3.4.3 +...+ 98.99.3

= 1.2.3 + 2.3.(4 - 1) + 3.4.(5 - 2) + ... + 98.99.(100 - 97)

= 1.2.3 + 2.3.4 - 1.2.3 + 3.4

5 - 2.3.4 + ... + 98.99.100 - 97.98.99

= 98.99.100

= 970200

Suy ra A = 970200 : 3

= 323400

Answer 2

A=1.2 +2.3+3.4+...+98.99

3A=1.2.3+2.3.3+3.4.3+...+98.99.3

3A=1.2.3+2.3.(4-1)+3.4.(5-2)+...+98.99.(100-97)

3A=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+98.99.100-97.98.99

3A=(1.2.3-1.2.3+2.3.4-2.3.4+...+97.98.99-97.98.99)+98.99.100

3A=98.99.100

A= 323400