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 ****