=-4+4-8+8+...-40+40=0

đúng nhé bạn

ta co tu suy nghi 

chuẩn CMNR ;+

S = 1 * 2 + 2 * 3 + 3 * 4 + ... + 38 * 39 + 39 * 40

=> 3S = 1 . 2 . 3 + 2 . 3 . 3 + 3 . 4 . 3 +... + 38 . 39 . 3 + 39 . 40 . 3

=> 3S = 1 . 2 . 3 + 2 . 3 . ( 4 - 1 ) + 3 . 4 . ( 5 - 2 ) + ... + 38 . 39 . ( 40 - 37 ) + 39 . 40 . ( 41 - 38 )

=> 3S = 1 . 2 . 3 + 2 . 3 . 4 - 1 . 2 .3 + 3 . 4. 5 - 2 . 3 .4 + ... + 38 . 39 . 40 - 37 . 38 . 39 + 39 . 40 . 41 - 38 . 39 . 40

=> 3S = 39 . 40 . 41

=> 3S = 63960

=> S = 21320

Vậy S là 21320

Ta có

S=1.2+2.3+....+39.40S=1.2+2.3+....+39.40

⇒3S=1.2(3−0)+2.3(4−1)+....+39.40(41−38)⇒3S=1.2(3−0)+2.3(4−1)+....+39.40(41−38)

⇒3S=1.2.3−0.1.2+2.3.4−1.2.3+....+39.40.41−38.39.40⇒3S=1.2.3−0.1.2+2.3.4−1.2.3+....+39.40.41−38.39.40

⇒ S = 39.40.41/3 ⇒ S = 39.40.41/3

=> S = 21320

Vậy S = 21320

A = \(\frac{2}{2\times3\times4}\) + \(\frac{2}{3\times4\times5}\) +...+\(\frac{2}{38\times39\times40}\)

A = \(\frac{4-2}{2\times3\times4}\) + \(\frac{5-3}{3\times4\times5}\) +...+\(\frac{40-38}{38\times39\times40}\)

A = \(\frac{4}{2\times3\times4}\) - \(\frac{2}{2\times3\times4}\) +\(\frac{5}{3\times4\times5}\) - \(\frac{3}{3\times4\times5}\) +...+\(\frac{40}{38\times39\times40}\) - \(\frac{38}{38\times39\times40}\)

A = \(\frac{1}{2\times3}\) - \(\frac{1}{3\times4}\) + \(\frac{1}{3\times4}\) - \(\frac{1}{4\times5}\) +...+\(\frac{1}{38\times39}\) - \(\frac{1}{39\times40}\)

A = \(\frac{1}{2\times3}\) - \(\frac{1}{39\times40}\)

A = \(\frac16\) - \(\frac{1}{1560}\)

A = \(\frac{260}{1560}\) - \(\frac{1}{1560}\)

A = 259/1560

* 100 là cái gì vậy .nhân à

79600 nha