Question

1.2+2.3+3.4+....+29.30

Answer 1

  A=1.2+2.3+3.4+....+29.30

3A=1.2.3+2.3.3+3.4.3+....+29.30.3

3A=1.2(3-0)+2.3(4-1)+3.4(5-2)+...+29.30(31-28)

3A=0.1.2-1.2.3+1.2.3-2.3.4+2.3.4-3.4.5+...+28.29.30-29.30.31

3A=29.30.31=26970

A=8990

Answer 2

A=8990

ko mình mình ko lại

Answer 3

A = 1.2 + 2.3 + 3.4 + ........+ 29.30

3A =1.2.3 + 2.3.3 + .... +29.30.3 

 3A   =1.2.3 + 2.3.(4-1)+ ......+ 29.30.(31-28)

3A = 1.2.3 + 2.3.4 - 1.2.3 +....-28.29.30

A = 28.29.30/3= 8120

Answer 4

mk nhầm các bạn ạ cho mk xin lỗi 

KQ = 8990