Ta có: $\vec{F}=\vec{F_{1}}+\vec{F_{2}}$
Với $(\vec{F_{1}};\vec{F_{2}})=\alpha$
$⇒|\vec{F}|=\sqrt{F_{1}²+F_{2}²+2.F_{1}.F_{2}.cos\alpha}$
Vì $cos\alpha∈[-1;1]$
$⇒\sqrt{F_{1}²+F_{2}²-2.F_{1}.F_{2}}≤|\vec{F}|≤\sqrt{F_{1}²+F_{2}²+2.F_{1}.F_{2}}$
$⇔|F_{1}-F_{2}≤|\vec{F}|≤|F_{1}+F_{2}|$
$⇔|16-12|≤|\vec{F}|≤|16+12|$
$⇔4N≤|\vec{F}|≤28N$