là sao???
 

15/(-x) = -3/4

-x.(-3) = 15.4

3x = 60

x = 60 : 3

x = 20

y/8 = -3/4

4y = 8.(-3)

4y = -24

y = -24 : 4

y = -6

\(\dfrac{x-2}{3}=\dfrac{x+1}{4}\)

=>4(x-2)=3(x+1)

=>4x-8=3x+3

=>x=11

\(x+\dfrac{1}{3}=\dfrac{25}{-5}\)

\(x+\dfrac{1}{3}=-5\)

\(x=-5-\dfrac{1}{3}\)

\(x=-\dfrac{16}{3}\)

x + 1/3 = 25/-5

x + 1/3 = -5

x             = -5 - 1/3

x             = -16/3

\(\dfrac{7}{8}+x=\dfrac{7}{20}\)

=>\(x=\dfrac{7}{20}-\dfrac{7}{8}=\dfrac{14}{40}-\dfrac{35}{40}=-\dfrac{21}{40}\)

=)x=7/20-7/8

=)x=-21/40

\(\left(-1,5\right)\cdot\dfrac{-2}{3}+\left(2,5-\dfrac{3}{4}\right):1\dfrac{3}{4}\)

\(=\dfrac{-3}{2}\cdot\dfrac{-2}{3}+\left(2,5-0,75\right):1,75\)

=1+1

=2

CM:A = \(\dfrac{1}{2^2}\) + \(\dfrac{1}{3^2}\) + \(\dfrac{1}{4^2}\) + ... + \(\dfrac{1}{100^2}\) < 1

   \(\dfrac{1}{2^2}\) = \(\dfrac{1}{2.2}\) < \(\dfrac{1}{1.2}\) = \(\dfrac{1}{1}-\dfrac{1}{2}\)

   \(\dfrac{1}{3^2}\) = \(\dfrac{1}{3.3}\) < \(\dfrac{1}{2.3}\) = \(\dfrac{1}{2}\) - \(\dfrac{1}{3}\)

    \(\dfrac{1}{4^2}\) = \(\dfrac{1}{4.4}\) < \(\dfrac{1}{3.4}\) = \(\dfrac{1}{3}\) - \(\dfrac{1}{4}\)

   \(\dfrac{1}{100^2}\) = \(\dfrac{1}{100.100}\) < \(\dfrac{1}{99.100}\) = \(\dfrac{1}{99}-\dfrac{1}{100}\)

Cộng vế với vế ta có:

    \(\dfrac{1}{2^2}\) + \(\dfrac{1}{3^2}\) + \(\dfrac{1}{4^2}\) + ... + \(\dfrac{1}{100^2}\)  = \(\dfrac{1}{1}\) - \(\dfrac{1}{100}\) < 1 (đpcm)