đoạn cuối: \(x\left(x-11\right)=12\Leftrightarrow x^2-11x=12\)
\(\Rightarrow x^2-11x+\)\(\dfrac{121}{4}=\dfrac{169}{4}\)
\(\Rightarrow\left(x^2-\dfrac{11}{2}\right)^2-\dfrac{169}{4}=0\)
\(\Rightarrow\left(x^2-\dfrac{11}{2}+\dfrac{13}{2}\right)\left(x^2-\dfrac{11}{2}-\dfrac{13}{2}\right)=0\)
\(\Rightarrow\left(x^2+1\right)\left(x^2-12\right)=0\)
\(\Rightarrow x^2-12=0\Leftrightarrow x=\pm\sqrt{12}\)
=>(x-4)(x-7)(x-5)(x-6)=1680
(x^2-11x+28)(x^2-11x+30)=1680
(x^2-11x+29-1)(x^2-11x+29+1)=1680
(x^2-11x+29)^2-1^2=1680
(x^2-11x+29)^2=1680+1=1681
<=>x^2-11x+29=41
x(x-11)=41-29=12
xet cac uoc cua 12 roi thay vao x , neu thoa man thi chinh la nghiem cua phuong trinh
Ta có : \(\left(x-4\right)\left(x-5\right)\left(x-6\right)\left(x-7\right)=1680\)
\(\Leftrightarrow\left(x^2-11x+28\right)\left(x^2-11x+30\right)-1680=0\)
Đặt \(t=\left(x^2-11x+28\right)\)
\(\Rightarrow t\left(t+2\right)-1680=0\)
\(\Rightarrow t^2+2t+1-1681=0\)
\(\Rightarrow\left(t+1\right)^2-41^2=0\)
\(\Rightarrow\left(t-40\right)\left(t-42\right)=0\)
\(\Rightarrow\left(x^2-11x-12\right)\left(x^2-11x-12\right)=0\)
\(\Rightarrow x\left(x-11\right)=12\)
Đến đây bạn tự giải nha