Ta có: \(\left(x-5\right)\left(x+5\right)-\left(x+3\right)^2+3\left(x-2\right)^2\)
\(=x^2-25-\left(x^2+6x+9\right)+3\left(x^2-4x+4\right)\)
\(=x^2-25-x^2-6x-9+3x^2-12x+12\)
\(=3x^2-18x-22\)
Ta có: \(\left(x+1\right)^2-\left(x-4\right)\left(x+4\right)+3x^2\)
\(=x^2+2x+1-\left(x^2-16\right)+3x^2\)
\(=4x^2+2x+1-x^2+16=3x^2+2x+17\)
Ta có: \(\left(x-5\right)\left(x+5\right)-\left(x+3\right)^2+3\left(x-2\right)^2=\left(x+1\right)^2-\left(x-4\right)\left(x+4\right)+3x^2\)
=>\(3x^2-18x-22=3x^2+2x+17\)
=>-20x=39
=>\(x=-\frac{39}{20}\)
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