Question

tinh gia tri bieu thuc: B=1*2*3+2*3*4+3*4*5+...+17*18*19

Answer 1

       B = 1.2.3 + 2.3.4 + 3.4.5 + .... + 17.18.19 

<=> 4B = 1.2.3.4 + 2.3.4.4 + 3.4.5.4 + .... + 17.18.19.4 

<=> 4B = 1.2.3.(4 - 0) + 2.3.4.(5 - 1) + 3.4.5.(6 - 2) + .... + 17.18.19.(20 - 16)

<=> 4B = 1.2.3.4 + 2.3.4.5 - 1.2.3.4 + .... + 17.18.19.20 - 16.17.18.19

<=> 4B = 17.18.19.20 = 116280

<=> B = 116280 : 4 = 29070 

Answer 2

       B = 1.2.3 + 2.3.4 + 3.4.5 + .... + 17.18.19 

<=> 4B = 1.2.3.4 + 2.3.4.4 + 3.4.5.4 + .... + 17.18.19.4 

<=> 4B = 1.2.3.(4 - 0) + 2.3.4.(5 - 1) + 3.4.5.(6 - 2) + .... + 17.18.19.(20 - 16)

<=> 4B = 1.2.3.4 + 2.3.4.5 - 1.2.3.4 + .... + 17.18.19.20 - 16.17.18.19

<=> 4B = 17.18.19.20 = 116280

<=> B = 116280 : 4 = 29070