$\angle BCD$ is an exterior angle of $\mathrm{\Delta }ABC.$ If $\angle BCD=3x°, \angle A=x°$ and $\angle B=80°,$ find the value of $x.$

 

Can the following lengths form the sides of a triangle? $($Yes$/$No$)$

$AB=4 \mathrm{cm}, BC=7 \mathrm{cm}\text{ and }CA=13 \mathrm{cm}$

Can the following lengths form the sides of a triangle? $($Yes$/$No$)$

$AB=9 \mathrm{cm}, BC=10 \mathrm{cm}\text{ and }CA=20 \mathrm{cm}$