`1)`
`a)(-91).11 `
`=(-91).(10+1) `
`=-910-91 `
`=-1001`
`b)221.(-21) `
`=221.(-20-1) `
`=-4420-221 `
`=-4641 `
`c)(-45).101 `
`=(-45).(100+1) `
`=-4500-45 `
`=-4545 `
`d)23.(-99)`
`=23.(-100+1)`
`=-2300+23`
`=-2277`
`2)`
`a) x(x-7)=0`
\(\to\left[ \begin{array}{l}x=0\\x-7=0\end{array} \right.\)
\(\to\left[ \begin{array}{l}x=0\\x=7\end{array} \right.\)
`b) x(x+11)=0`
\(\to\left[ \begin{array}{l}x=0\\x+11=0\end{array} \right.\)
\(\to\left[ \begin{array}{l}x=0\\x=-11\end{array} \right.\)
`c)(x+8)(x-12)=0`
\(\to\left[ \begin{array}{l}x+8=0\\x-12=0\end{array} \right.\)
\(\to\left[ \begin{array}{l}x=-8\\x=12\end{array} \right.\)
`d)(x-3)(x^2+3)=0`
\(\to\left[ \begin{array}{l}x-3=0\\x^2+3=0\end{array} \right.\)
\(\to\left[ \begin{array}{l}x=3\\x^2=-3\quad(\text{vô lí})\end{array} \right.\)
