Đáp án:
Giải thích các bước giải:
\(\begin{array}{l}{\left( {10a + 5} \right)^2} = {\left( {10a} \right)^2} + 2.10a.5 + 25 = 100{a^2} + 100a + 25\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 100a\left( {a + 1} \right) + 25\\\,\, \Rightarrow {25^2} = 100.2\left( {2 + 1} \right) + 25 = 625\,\,;\\\,\,\,\,\,\,\,\,\,{35^2} = 100.3\left( {3 + 1} \right) + 25 = 1225\,\,;\\\,\,\,\,\,\,\,\,\,{65^2} = 100.6\left( {6 + 1} \right) + 25 = 4225\,\,;\\\,\,\,\,\,\,\,\,\,{75^2} = 100.7\left( {7 + 1} \right) + 25 = 5625.\end{array}\)
