c) \(\left(x+3\right)^2-\left(x-6\right)^2=3x-1\)
\(\Leftrightarrow x^2+6x+9-\left(x^2-12x+36\right)=3x-1\)
\(\Leftrightarrow x^2+6x+9-x^2+12x-36=3x-1\)
\(\Leftrightarrow18x-27=3x-1\)
\(\Leftrightarrow18x-3x=-1+27\)
\(\Leftrightarrow15x=26\)
\(\Leftrightarrow x=\dfrac{26}{15}\)
d) \(\left(5x+1\right)\left(5x-1\right)-\left(2-5x\right)^2=3\)
\(\Leftrightarrow\left(5x\right)^2-1^2-\left(4-20x+25x^2\right)=3\)
\(\Leftrightarrow25x^2-1-4+20x-25x^2=3\)
\(\Leftrightarrow20x-5=3\)
\(\Leftrightarrow20x=3+5\)
\(\Leftrightarrow20x=8\)
\(\Leftrightarrow x=\dfrac{8}{20}\)
\(\Leftrightarrow x=\dfrac{2}{5}\)
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