\(5^x+5^{x+1}+5^{x+2}=775\)

\(\Leftrightarrow5^x.1+5^x.5+5^x.5^2=775\)

\(\Leftrightarrow5^x.\left(1+5+5^2\right)=775\)

\(\Leftrightarrow5^x.\left(1+5+25\right)=775\)

\(\Leftrightarrow5^x.31=775\)

\(\Rightarrow5^x=775:31=25=5^2\)

\(\Rightarrow x=2\)

Vậy \(x=2\)

5^x + 5^{x + 1} + 5^{x + 2} = 775

5^x . 1 + 5^x . 5 + 5^x . 25 = 775

5^x . (1 + 5 + 25) = 775

5^x . 31 = 775

5^x       = 775 : 31

5^x       = 25

5^x       = 5²

Suy ra : x = 2

muon 1ab5c chia het cho 775 thi 1ab5c phai chia het cho 31 va 25

muon 1ab5c chia het cho 25 thi 5c phai chia het cho 25 nen c=0

Vi 1ab50 chia het cho 31 nen a=0:b=8:c=0

a=3:b=9:c=0

a=7:b=0:c=0

5^x + 5^{x + 1} + 5^{x + 2} = 775

\(\Rightarrow\) 5^x + 5^{x + 1} + 5^{x + 2} = 5² + 5^{2 + 1} + 5^{2 + 2}

\(\Rightarrow\) x = 2

5^x + 5^x+1 + 5^x+2 = 775

5^x . 1 + 5^x . 5 + 5^x + 5² = 775

5^x . (1 + 5 + 25 ) = 775

5^x . 31 = 775

5^x = 775 : 31

5^x = 25

5^x = 5²

^x=2

\(\frac{1}{7}+\frac{1}{91}+\frac{1}{247}+\frac{1}{475}+\frac{1}{775}+\frac{1}{1147}\)

\(=\frac{1}{1\cdot7}+\frac{1}{7\cdot13}+\frac{1}{13\cdot19}+...+\frac{1}{31\cdot37}\)

\(=\frac{1}{6}\left(\frac{6}{1\cdot7}+\frac{6}{7\cdot13}+\frac{6}{13\cdot19}+...+\frac{6}{31\cdot37}\right)\)

\(=\frac{1}{6}\left(1-\frac{1}{7}+\frac{1}{7}-\frac{1}{13}+\frac{1}{13}-\frac{1}{19}+...-\frac{1}{31}-\frac{1}{37}\right)\)

\(=\frac{1}{6}\left(1-\frac{1}{37}\right)\)

\(=\frac{1}{6}\cdot\frac{36}{37}=\frac{6}{37}\)

Tổng cần tính bằng:\(\frac{1}{1.7}\)+\(\frac{1}{7.13}\)+\(\frac{1}{13.19}\)+\(\frac{1}{19.25}\)+\(\frac{1}{25.31}\)+\(\frac{1}{31.37}\)=(\(\frac{1}{1}\)-\(\frac{1}{7}\)+\(\frac{1}{7}\)-\(\frac{1}{13}\)+...+\(\frac{1}{31}\)\(\frac{1}{37}\)):3 =(\(1\)-\(\frac{1}{37}\)):3=\(\frac{12}{37}\)

a) Ta có : -3535/-3737 = -35/-37

=> 35/37 > -35/-37

A=1/7 +1/91 +1/247 + 1/475 + 1/775 + 1/1147

A=1/(1.7)+1/(7.13)+1/(13.19)+...+1/(31...  

A=(1/6)*( 1 - 1/7 + 1/7 - 1/13 +... +1/31-1/37)  

A=(1/6)*(1-1/37)  

A=(1/6)*(36/37)  

A=6/37 

6/37 đúng đó

Ai tích mk mk sẽ tích cho

=\(\frac{6}{37}\)

\(S=\frac{5^2}{1.6}+\frac{5^2}{6.11}+...+\frac{5^2}{26.31}\)

\(S=5.\left(\frac{5}{1.6}+\frac{5}{6.11}+...+\frac{5}{26.31}\right)\)

\(S=5.\left(1-\frac{1}{6}+\frac{1}{6}-\frac{1}{11}+...+\frac{1}{26}-\frac{1}{31}\right)\)

\(S=5.\left(1-\frac{1}{31}\right)\)

\(S=5.\frac{30}{31}\)

\(S=\frac{150}{31}\)

Câu L bạn thiếu số\(\frac{1}{475}\)

\(L=\frac{1}{7}+\frac{1}{91}+\frac{1}{247}+\frac{1}{475}+\frac{1}{775}+\frac{1}{1147}\)

\(L=\frac{1}{1.7}+\frac{1}{7.13}+\frac{1}{13.19}+\frac{1}{19.25}+\frac{1}{25.31}+\frac{1}{31.37}\)

\(L=\frac{1}{6}.\left(\frac{6}{1.7}+\frac{6}{7.13}+\frac{6}{13.19}+\frac{6}{19.25}+\frac{6}{25.31}+\frac{6}{31.37}\right)\)

\(L=\frac{1}{6}.\left(1-\frac{1}{7}+\frac{1}{7}-\frac{1}{13}+\frac{1}{13}-\frac{1}{19}+\frac{1}{19}-\frac{1}{25}+\frac{1}{25}-\frac{1}{31}+\frac{1}{31}-\frac{1}{37}\right)\)

\(L=\frac{1}{6}.\left(1-\frac{1}{37}\right)\)

\(L=\frac{1}{6}.\frac{36}{37}\)

\(L=\frac{6}{37}\)

\(S=\frac{5^2}{1.6}+\frac{5^2}{6.11}+...+\frac{5^2}{26.31}\)

\(=5\left[\left(1-\frac{1}{6}\right)+\left(\frac{1}{6}-\frac{1}{11}\right)+...+\left(\frac{1}{26}-\frac{1}{31}\right)\right]\)

\(=5\left[1-\frac{1}{31}\right]\)

\(=5.\frac{30}{31}=\frac{150}{31}\)

\(L=\frac{1}{7}+\frac{1}{91}+\frac{1}{247}+\frac{1}{775}+\frac{1}{1147}\)

\(L.6=\frac{6}{1.7}+\frac{6}{7.13}+\frac{6}{13.19}+\frac{6}{19.25}+\frac{6}{25.31}\)

\(L.6=\left(1-\frac{1}{7}\right)+\left(\frac{1}{7}-\frac{1}{13}\right)+\left(\frac{1}{13}-\frac{1}{19}\right)+\left(\frac{1}{19}-\frac{1}{25}\right)+\left(\frac{1}{25}-\frac{1}{31}\right)\)

\(L.6=1-\frac{1}{31}\)

\(L.6=\frac{31}{31}-\frac{1}{31}\)

\(L.6=\frac{25}{31}\)

\(L=\frac{30}{31}:6\)

\(L=\frac{30}{31}.\frac{1}{6}\)

\(L=\frac{30}{186}\)