Tìm x: a; 20-[4 mũ 2+(x-6)]=90 b;(x+3).(2.x-4)=o c;1000:[30+(2-6)]= 3 mũ 2 +4 mũ 2 và x thuộc N

`a,`

`20-[4^2+(x-6)]=90`

`=>20-[16+x-6]=90`

`=>20-[10+x]=90`

`=>20-10-x=90`

`=>10-x=90`

`=>x=10-90`

`=>x=-80`

Vậy `x=-80`

$\\$

`b,`

`(x+3).(2x-4)=0`

`=>`\(\left[ \begin{array}{l}x+3=0\\2x-4=0\end{array} \right.\) 

`=>`\(\left[ \begin{array}{l}x=-3\\2x=4\end{array} \right.\) 

`=>`\(\left[ \begin{array}{l}x=-3\\x=2\end{array} \right.\) 

Vậy `x=-3; x=2`

$\\$

`c,`

`1000:[30+(2^x-6)]=3^2+4^2`

`=>1000:[30+2^x-6]=9+16`

`=>1000:(24+2^x)=25`

`=>24+2^x=1000:25`

`=>24+2^x=40`

`=>2^x=40-24`

`=>2^x=16`

`=>2^x=2^4`

`=>x=4`

Vậy `x=4`