1,Tìm x thuộc Z biết:
a, x.(x+3)=0 \(\Leftrightarrow\left[{}\begin{matrix}x=0\\x+3=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=0\\x=-3\end{matrix}\right.\)
Kl: x=0, x=-3
b, (x-2).(5-x)=0 \(\Leftrightarrow\left[{}\begin{matrix}x-2=0\\5-x=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=2\\x=5\end{matrix}\right.\)
Kl: x=2, x=5
c, (x-1).(x2+1)=0 \(\Leftrightarrow x-1=0\) (vì \(x^2+1>0\))
\(\Leftrightarrow x=1\)
Kl: x=1
2,cho A=(5m2-8m2-9m2).(-n2+4n3)
Với giá trị nào của m,n thì A>0
\(A>0\Leftrightarrow\left(5m^2-8m^2-9m^2\right)\left(-n^2+4n^3\right)>0\)
\(\Leftrightarrow-12m^2\left(-n^2+4n^3\right)>0\)
\(\Leftrightarrow-12m^2\cdot\left(-n^2\right)\left(1-4n\right)>0\)
\(\Leftrightarrow12m^2n^2\left(1-4n\right)>0\)
\(\Leftrightarrow1-4n>0\)
\(\Leftrightarrow n< \dfrac{1}{4}\)
kl: A > 0 \(\Leftrightarrow\left\{{}\begin{matrix}m,n\in R\\n< \dfrac{1}{4}\end{matrix}\right.\)
3,Tìm x biết:
a, -12.(x-5) + 7.(3-x)=5 \(\Leftrightarrow-12x+60+21-7x=5\Leftrightarrow-19x=-76\Leftrightarrow x=4\)
Kl: x=4
b,30.(x+2)-6(x-5)-24x = 100 \(\Leftrightarrow30x+60-6x+30-24x=100\Leftrightarrow0x=10\left(VN\right)\)
Kl: ptvn