A body travels a distance of 3 km towards East, then 4 km towards North and finally 9 km towards East. What is the resultant displacement ?

 

In the above diagram, the journey of the boy has been represented.

AC  is the displacement.

And to find the value of the displacement $(AC)$ let us use Pythagoras theorem in $\vartriangle ABC$

$AC^2=AB^2+BC^2$

Or $AC^2=(9+3)^2+4^2$

Or $AC^2=12^2+4^2$

Or $AC^2=144+16$

Or $AC^2=160$

Or $AC=\sqrt{160}$

Or $AC=12.5\ km$