Let [tex] x,y [/tex] be the lengths of the original legs. The original area is thus
[tex] \dfrac{xy}{2} [/tex]
Now, we triple the legs, so we have [tex] (x,y)\mapsto(3x,3y) [/tex]
The new area is
[tex] \dfrac{(3x)(3y)}{2} = \dfrac{9xy}{2} = 9\dfrac{xy}{2} [/tex]
which is 9 times the original area.
So, if you triple both legs, the area will be nine times as much as the original one.
