Question

Tính a=1x2+2x3+3x4+...+99x100

Answer 1

A = 1.2+2.3+3.4+......+99.100 
Gấp A lên 3 lần ta có: 
A . 3 = 1.2.3 + 2.3.3 + 3.4.3 + … + 99.100.3 
A . 3 = 1.2.3 + 2.3.(4 - 1) + 3.4.( 5 - 2) + … + 99.100. (101 - 98) 
A . 3 = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + … + 99.100.101 - 98.99.100 
A . 3 = 99.100.101 
A = 99.100.101 : 3 
A = 33.100.101 
A = 333 300

Answer 2

bạn giải được không

Answer 3

3A=3.(1x2+2x3+3x4+....+99x100)

3A=3x1x2x+3x2x3+3x3x4+...+3x99x100

3A=(3-0)x1x2+(4-1)x2x3+(5-2)x3x4+...+(101-98)x99x100

3A=1x2x3-0+2x3x4-1x2x3+3x4x5-2x3x4+...+99x100x101-98x99x100

3A=99x100x101-0

3A=99x100x101

A=33x100x101

A=333 300