Question

Tính 

A = 1.2 + 2.3 + 3.4 + ............ + 99.100

Answer 1

số số hạng : (99,100 -1.2) : 1.1 +1=90 số 

Tổng: (99.100 +1.2) x 90 : 2= 4513 ,5

Answer 2

\(A=1.2+2.3+3.4+....+99.100\\ 3.A=1.2.3+2.3.3+....+99.100.3\)

\(3.A=1.2.3+2.3.\left(4-1\right)+....+99.100\left(101-98\right)\\ 3.A=1.2.3+.....+99.100.101-98.99.100\)

\(3.A=99.100.101\\ A=33.100.101=333300\)

Answer 3

A=1.2+2.3+3.4+....+99.100

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

3A=1.2.3+2.3(4-1)+3.4.(5-2)+.....+99.100.(101-98)

3A=1.2.3+2.3.4-2.3.1+...+98.99.100-99.100.101

3A=99.100.101

=>A=\(\frac{99.100.101}{3}\)

=>A=33300 hoặc 333300 lấy casio fx570 mà tính