\(S=\dfrac{1}{3.5}+\dfrac{1}{5.7}+\dfrac{1}{7.9}+...+\dfrac{1}{97.99}\)

\(2S=\dfrac{2}{3.5}+\dfrac{2}{5.7}+\dfrac{2}{7.9}+...+\dfrac{2}{97.99}\)

\(2S=\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+...+\dfrac{1}{97}-\dfrac{1}{99}\)

\(2S=\dfrac{1}{3}-\dfrac{1}{99}\)

\(2S=\dfrac{32}{99}\)

\(S=\dfrac{32}{99}:2\)

\(S=\dfrac{16}{99}\)

\(76,22-y\times3=61,22\)

                  \(y\times3=76,22-61,22\)

                   \(y\times3=15\)

                            \(x=15:3\)

                            \(x=5\)

76,22 - y * 3 = 30,61 * 2

76,22 - y * 3 = 61,21

     76,22 - y = 61,22 : 3 

     76,22 - y = 20/61/150

                y = 76,21 - 20/61/150

                y = 55/241/300

\(76,22-y\times3=30,61\times2\)

\(\Leftrightarrow\)\(76,22-y\times3=61,22\)

\(\Leftrightarrow\)\(y\times3=76,22-61,22\)

\(\Leftrightarrow\)\(y\times3=15\)

\(\Leftrightarrow\)\(y=15\div3\)

\(\Leftrightarrow\)\(y=5\)

Ta có : \(S=\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{97.99}\)

\(\Rightarrow2S=2\left(\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{97.99}\right)\)

\(\Rightarrow2S=\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{97.99}\)

\(\Rightarrow2S=\left(\frac{1}{3}-\frac{1}{5}\right)+\left(\frac{1}{5}-\frac{1}{7}\right)+...+\left(\frac{1}{97}-\frac{1}{99}\right)\)

\(\Rightarrow2S=\left(\frac{1}{3}-\frac{1}{99}\right)+\left[\left(\frac{1}{5}+...+\frac{1}{97}\right)-\left(\frac{1}{5}+\frac{1}{7}+...+\frac{1}{97}\right)\right]\)

\(\Rightarrow2S=\left(\frac{33}{99}-\frac{1}{99}\right)+0\)

\(\Rightarrow2S=\frac{32}{99}\)

\(\Rightarrow S=\frac{32}{99}\div2\)

\(\Rightarrow S=\frac{16}{99}\)

\(S=\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+...+\frac{1}{97.99}\)

\(S=\frac{1}{2}.\left(\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{97.99}\right)\)

\(S=\frac{1}{2}.\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{97}-\frac{1}{99}\right)\)

\(S=\frac{1}{2}.\left(\frac{1}{3}-\frac{1}{99}\right)\)

\(S=\frac{1}{2}.\frac{32}{99}=\frac{16}{99}\)                

1: =>0,5:(0,4-x)=2,5

=>0,4-x=0,5/2,5=0,5

=>x=0,4-0,5=-0,1

2: =>x*(3,6+6,4)=18,9

=>10x=18,9

=>x=1,89

3: =>(9,4+5,3):x=14

=>14,7:x=14

=>x=14,7/14=1,05

c=1.5.6+2.10.12+4.20.24+9.45.54:1.3.5+2.6.10+4.12.20+9.27.45

C=1.5.6+1.5.6.8+1.5.6.64+1.5.6.729:1.3.5+1.3.5.8+1.3.5.64+1.3.5.729

C=1.5.6(1+8+64+729):1.3.5(1+8+64+729)=1.5.6:1.3.5=2

x+x.3/4=1

x(1+3/4)=1

x+7/4=1

x=4/7

bạn làm dễ hiểu hoưn đươcj ko

( 9 * 8 - 12 * 5 * 12 ) * ( 1 + 2 + 3 + 4 + 5 )

= ( 72 - 720 ) * 15

= ( -- 648 ) * 15

= -9720