+ $(\vec{F_1}, \vec{F_2})=0°$
$F_{12}=F_1+F_2=16+12=28N$
+ $(\vec{F_1}, \vec{F_2})=60°$
$F_{12}=\sqrt{F_1^2+F_2^2+2F_1F_2cos\alpha}$
$=\sqrt{16^2+12^2+2.16.12.cos60}=4\sqrt{37}N$
+ $(\vec{F_1}, \vec{F_2})=120°$
$F_{12}=\sqrt{F_1^2+F_2^2+2F_1F_2cos\alpha}$
$=\sqrt{16^2+12^2+2.16.12.cos120}=4\sqrt{13}N$
+ $(\vec{F_1}, \vec{F_2})=180°$
$F_{12}=|F_1-F_2|=|16-12|=4N$
Góc hợp bởi hai hợp lực để F12=20N:
$F_{12}=\sqrt{F_1^2+F_2^2+2F_1F_2cos\alpha}$
$\Leftrightarrow20=\sqrt{16^2+12^2+2.16.12.cos\alpha}$
$\Rightarrow cos \alpha=0$
$\Rightarrow \alpha=90°$