Question

jup toi giai bai nay voi 1x2+2x3+3x4+4x5+......+9x10

 

Answer 1

\(1.2+2.3+3.4+4.5+...+9.10\)

Đặt A = \(1.2+2.3+3.4+4.5+...+9.10\)

\(3.A\) = \(1.2.3+2.3.3+3.4.3+4.5.3+...+9.10.3\)

=> \(3.A\) = \(1.2.3+2.3.4+3.4.5+4.5.6+...+9.10.11\)

Ta có: \(3.A=9.10.11\)

=>: \(A=\frac{9.10.11}{3}=330\)

=> A = 330 

Answer 2

2+6+12+20+30+42+56+72+90=330

Answer 3

\(1.2+2.3+4.5+...+9.10\)

Dat A=1.2+2.3+3.4+4.5+...+9.10

3.A=1.2.3+2.3.3+3.4.3+4.5.3+...+9.10.3

3A=1.2.3+2.3.4+3.4.5+4.5.6+...+9.10.11

Ta co:3A=9.10.11

A=9.10.11:3

A=330

Vay A=330

Answer 4

trả lời 

=330 

chúc bn 

hc tốt