Ta có: \(A=-\frac{4}{20}-\frac{4}{30}-\frac{4}{42}-\frac{4}{56}-\frac{4}{72}-\frac{4}{90}\)

   \(\Leftrightarrow A=-4.\left(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}\right)\)

   \(\Leftrightarrow A=-4.\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\right)\)

   \(\Leftrightarrow A=-4.\left(\frac{1}{4}-\frac{1}{10}\right)\)

   \(\Leftrightarrow A=-4.\left(\frac{5-2}{20}\right)\)

   \(\Leftrightarrow A=-4.\frac{3}{20}=-\frac{3}{5}\)

Vậy \(\Leftrightarrow A=-\frac{3}{5}\)

thanks

A=-4/4.5+-4/5.6+-4/6.7+-4/7.8+-4/8.9+-4/9.10

-1/4A=1/4.5+1/5.6+1/6.7+1/7.8+1/8.9+1/9.10

-1/4A=1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10

-1/4A=1/4-1/10

-1/4A=3/20

A=-3/5

thanks

Ta có : A = -4/20+-4/30+-4/42+-4/56+-4/72+-4/90

                = -4/4.5+-4/5.6+-4/6.7 +-4/7.8+-4/8.9+-4/9.10

                 =-4/4-(-4/5)+(-4/5)-(-4/6)+......+(-4/9)-(-4/10)

                  =-4/4-(-4/10)

                 =-3/5

# chúc bạn học tốt #

\(A=\frac{-4}{20}+\frac{-4}{30}+\frac{-4}{42}+...+\frac{-4}{90}\)

     =\(-4\left(\frac{1}{20}+\frac{1}{30}+...+\frac{1}{90}\right)\)

    =-4\(\left(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{9.10}\right)\)

  =-4\(\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{9}-\frac{1}{10}\right)\)

=-4\(\left(\frac{1}{4}-\frac{1}{10}\right)\)

=-4.\(\frac{3}{20}\)=\(\frac{-3}{5}\)

Đổi \(\frac{1}{4}\)h = 15 phút

       \(\frac{1}{3}\)h = 20 phút

         \(\frac{1}{5}\)h = 12 phút

\(\Rightarrow\)Người thứ ba là người đến B đầu tiên.

k mk nha

syntax ERROR bạn ơi

81x^4+4

=81x^4+4+36x^2-36x^2

=(9x^2+2)^2-36x^2

=(9x^2+6x+2)(9x^2-6x+2)

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