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A = 25^28 + 25^24 + .... + 25^4 + 1
25^4.A = 25^32 + 25^28 + 25^24 + ...+ 25^4
=> (25^4 - 1)A = 25^32 - 1
=> A = (25^32 - 1)/(25^4 - 1)
B = 25^30 + 25^28 + ... + 25^2 + 1
25^2.B = 25^32 + 25^30 + 25^28 + ...+ 25^2
=> (25^2 - 1)B = 25^32 - 1
=> B = (25^32 - 1)/(25^2 - 1)
vậy :
A/B = [(25^32 - 1)/(25^4 - 1)]/[(25^32 - 1)/(25^2 - 1)] = (25^2 - 1)/(25^4 -1) = 1/(25^2 + 1) = 1/626
Địa chỉ chi tiết : yahoo.com
C = \(\frac{25^{28}+25^{24}+...+25^4+25^0}{25^{30}+25^{28}+...+25^2+25^0}\)
= \(\frac{25^{28}+25^{24}+...+25^2+25^0}{\left(25^{30}+25^{26}+...+25^2\right)+\left(25^{28}+25^{24}+...+25^0\right)}\)
= \(\frac{25^{28}+25^{24}+...+25^0}{25^2\left(25^{28}+25^{24}+...+1\right)+\left(25^{28}+25^{24}+...+1\right)}\)
= \(\frac{25^{28}+25^{24}+...+1}{\left(25^2+1\right)\left(25^{28}+25^{24}+...+1\right)}=\frac{1}{626}\)
Cho 10 ****
\(x-25\%\times x=\frac{1}{2}\)
\(x-\frac{25}{100}\times x=\frac{1}{2}\)
\(x\times\left(1-\frac{25}{100}\right)=\frac{1}{2}\)
\(x\times\frac{3}{4}=\frac{1}{2}\)
\(x=\frac{1}{2}\div\frac{3}{4}\)
\(x=\frac{2}{3}\)
a,1/5+2/5+3/5+4/5+...+9/5
=(1+2+3+4+...+9)/5
=45/5
=9
b,17,8(3,7+5,7)-7,8(4,6+4,8)
=17,8.9,4-7,8.9,4
=9,4(17,8-7,8)
=9,4.10
=94
pt \(\Leftrightarrow\left(\frac{x-2}{27}-1\right)+\left(\frac{x-3}{26}-1\right)+\left(\frac{x-4}{25}-1\right)+\left(\frac{x-5}{24}-1\right)+\left(\frac{x-44}{5}+3\right)=0\)
\(\Leftrightarrow\frac{x-29}{27}+\frac{x-29}{26}+\frac{x-29}{25}+\frac{x-29}{24}+\frac{x-29}{5}=0\)
\(\Leftrightarrow\left(x-29\right)\left(\frac{1}{27}+\frac{1}{26}+\frac{1}{25}+\frac{1}{24}+\frac{1}{5}\right)=0\)
Mà \(\frac{1}{27}+\frac{1}{26}+\frac{1}{25}+\frac{1}{24}+\frac{1}{5}\ne0\)
\(\Rightarrow x-29=0\Leftrightarrow x=29\)
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