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

1/1/3+3/1/4+4/1/5

=4/3+13/4+21/5

=?

1. $(x-3)^2=0$

$x-3=0$

$x=3$

Vậy $x=3$.

2. $(2x+1)^2=4$

$2x+1=\pm2$

$2x+1=2\Rightarrow x=\frac12$

$2x+1=-2\Rightarrow x=-\frac32$

Vậy $x=\frac12;-\frac32$.

\(\text{S = 1 x 2 +2x3 + 3x4+... + 38x39+39x40 }\)

\(\text{3S= 1.2.3+2.3.3+3.4.3+...+38.39.3+39.40.3}\)

\(\text{3S=1.2.(3-0)+2.3(4-1)+...+38.39(40-37)}\)

\(\text{3S=0.1.2-1.2.3+1.2.3-2.3.4+...+37.39.39-38.39.40}\)

\(\text{3S=38.39.40 S=19760 }\)

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

=> S = 19760 

a) 1/7 . 5/6 + 1/7 . 1/6 + -8/7

= 1/7 . ( 5/6 + 1/6 ) + -8/7

= 1/7 . 1 + -8/7

= 1/7 + -8/7 = -1

b) 3/5 . -4/9 . 5/3 . 18/7

= ( 3/5 . 5/3 ) . ( -4/9 . 18/7 ) 

= 1 . -8/7 = -8/7

c) 1/2 + -3/4 . 16/9

= 1/2 + -4/3 = -5/6

d) ( 1/7 + 5/14 ) . -28/3

= 1/2 . -28/3 = -14/3

tính như bth thôi ráng lên bạn nhá

99/32

qua de dang nhe

S=1/(1+2)+1/(1+2+3)+1/(1+2+3+4)+...+1/(1+2+3+4+...+10)

S=1/(2*3/2)+1/(3*4/2)+1/(4*5/2)+...+1/(10*11/2)

S=2(1/(2*3)+1/(3*4)+1/(4*5)+1/(5*6)+...+1/(10*11)

S=2(1/2-1/3+1/3-1/4+1/4-1/5+...+1/10-1/11)

S=2(1/2-1/11)

S=2*9/22

S=9/11

nho k cho minh voi nha