Question

cos120°=-0,50
a=10N b=15N 0=120°

Answer 1

$F^2 = F_{1}^2 + F_{2}^2 - 2.cos \theta .F_{1}.F_{2} \\ F^2 = 10^2 + 15^2 -2.(-0,5).10.15 \\ F^2 = 100 + 225 + 150 = 475$
$F$≈21,8N