a) $3^{x+1}+3^{x+2}+3^{x+3}+..+3^{x+100} $
$=3^x.(3^1+3^2+...+3^{100}) $
=3^x.[(3+3^2+3^3+3^4)+...+(3^{97}+3^{98}+3^{99}+3^{100})] $
$=3^x.(3+3^2+3^3+3^4).(1+...+3^{97})$
$=3^x.120.(1+...+3^{97}) \vdots 120$
b) Áp dung tính chất dãy tỉ số bằng nhau có :
$\dfrac{12x-8y+6z-12x+8y-6z}{4^2+3^2+2^2}= 0 $
$⇒3x-2y=0,2x-4z=0,4y-3z=0$
$⇒3x=2y,2x=4z,4y=3z$
$⇒ \dfrac{x}{2} = \dfrac{y}{3} = \dfrac{z}{4}$