Bài 3:
\(2,25m^2+0,36n^2-1,8mn\\ =\left(1,5m\right)^2-2.1,5.0,6.m.n+\left(0,6n\right)^2\\ =\left(1,5m-0,6n\right)^2\)
Bài 2:
\(\left(x+7\right)^2-\left(x+3\right).\left(x-3\right)=0\\ \Leftrightarrow x^2+14x+49-\left(x^2-9\right)=0\\ \Leftrightarrow14x=-58\\ \Leftrightarrow x=\dfrac{-58}{14}=\dfrac{-29}{7}\)