Tính hợp lí:       \[\left( { - 25} \right) \cdot \left( { - 42} \right) - 125 \cdot 42\]

b1) Ta có        \[x + \left( { - 5} \right) = 13\]

                             \[x\;\;\;\;\;\;\;\;\;\;\; = {\rm{ }}13{\rm{ }}--{\rm{ }}\left( { - 5} \right)\]

                             \[x\;\;\;\;\;\;\;\;\;\;\; = {\rm{ }}18\]

b2) \[x\] thuộc Ư(10) \[ = \left\{ {1\,;\,\,2\,;\,\,5\,;\,\,10\,;\,\, - 1\,;\,\, - 2\,;\,\, - 5\,;\,\, - 10} \right\}\]

Ta có \[A = {5^3} + {5^4} + {5^5} + \ldots + {5^{10}}\]

\[ = \left( {{5^3} + {\rm{ }}{5^4}} \right){\rm{ }} + {\rm{ }}\left( {{5^5} + {\rm{ }}{5^6}} \right){\rm{ + }}... + {\rm{ }}\left( {{5^9} + {\rm{ }}{5^{10}}} \right)\]

\[ = {5^2}\left( {5 + {5^2}} \right) + {5^4}\left( {5 + {5^2}} \right) + ... + {5^8}\left( {5 + {5^2}} \right)\]

\[ = 30 \cdot \left( {{5^2} + {5^4} + ... + {5^8}} \right)\,\,\, \vdots \,\,\,30\].