Show that the area of the triangle contained between the vectors `veca and vecb` is one half of the magnitude of `vecaxxvecb`.

Explanation

In `DeltaOAB, vec(OA)=vecb, vec(OB)=veca` and `angleAOB=theta and AC=h`
`:.` from `DeltaACO`
`(AC)/(OA)=sintheta`
`:. H=bsintheta`
Now area of `DeltaOAB=(1)/(2)xx|vec(OB)|h`
`=(1)/(2)|veca|h`
`:.` Area = `(1)/(2)absintheta [because h=bsintheta]`
`=(1)/(2)|veca||vecb|sinthetahatn`
`:.vecS=(1)/(2)(vecaxxvecb)hatn`
Magnitude `S=(1)/(2)|vecaxxvecb|`