Question

TÍNH S :1*2+2*3+3*4+...+28*29+29*30

Answer 1

S = 1 * 2 + 2 * 3 + 3 * 4 + ...... + 28 * 29 + 29 * 30

3S = 1 x 2 x 3 - 1 x 2 x 3 + 2 x 3 x 4 - 2 x 3 x 4 + .............. + 29 x 30 x 31

3S = 29 x 30 x 31

S = 29 x 30 x 31 : 3 = 8990

Answer 2

Ta có : S = 1.2 + 2.3 + 3.4 + ..... + 28.29 + 29.30

<=> 3S = 1.2.3 - 1.2.3 + 2.3.4 - 2.3.4 + ...... + 29.30.31

<=> 3S = 29.30.31 

<=> S = 29.30.31 / 3 = 8990

Chúc học tốt nhé !

Answer 3

s = 1.2 + 2.3 + 3.4 +...+ 29.29 + 29.30

=> 3S = 1.2.3 - 1.2.3 + 2.3.4 - 2.3.4 +...+ 29.30.31

3S = 29.30.31

S = 29.30 . 31/3 = 8990