a) \(x\left(5-2x\right)+2x\left(x-1\right)=15\)
\(\Leftrightarrow5x-2x^2+2x^2-2x=15\)
\(\Leftrightarrow3x=15\Leftrightarrow x=5\)
b) \(5x\left(12x+7\right)-3x\left(20x-5\right)=-100\)
\(\Leftrightarrow60x^2+35x-60x^2+15x=-100\)
\(\Leftrightarrow50x=-100\Leftrightarrow x=-2\)
c) \(\left(x^2-2x+1\right).3+\left(2-3x\right)x=7\)
\(\Leftrightarrow3x^2-6x+3+2x-3x^2=7\)
\(\Leftrightarrow-4x=4\Leftrightarrow x=-1\)
d) \(3\left(5x-1\right)-x\left(x+1\right)+x^2=14\)
\(\Leftrightarrow15x-3-x^2-x+x^2=14\)
\(\Leftrightarrow14x=17\Leftrightarrow x=\dfrac{17}{14}\)
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