a: =>(x+3)(5x+7)-(x+3)(x-3)=0
=>(x+3)(5x+7-x+3)=0
=>(x+3)(4x+10)=0
=>x=-5/2 hoặc x=-3
b: =>(x+5)(2x+1-x-5)=0
=>(x+5)(x-4)=0
=>x=4 hoặc x=-5
d: \(\Leftrightarrow\left(x^2-9x+14\right)\left(x^2-9x+20\right)=72\)
\(\Leftrightarrow\left(x^2-9x\right)^2+34\left(x^2-9x\right)+280-72=0\)
\(\Leftrightarrow\left(x^2-9x\right)^2+34\left(x^2-9x\right)+208=0\)
\(\Leftrightarrow\left(x^2-9x-8\right)\left(x^2-9x-26\right)=0\)
hay \(x\in\left\{\dfrac{9+\sqrt{113}}{2};\dfrac{9-\sqrt{113}}{2};\dfrac{9+\sqrt{185}}{2};\dfrac{9-\sqrt{185}}{2}\right\}\)
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