Using slopes, prove that the points `A(-2,-1)`, `B(1,0)`, `C(4,3)` and `D(1,2)` are the vertices of a parallelogram.

Explanation

Slope of `AB=(0-(-1))/(1-(-2))=(1)/(3)`
Slope of `BC=(3-0)/(4-1)=1`
Slope of `CD=(2-3)/(1-4)=(1)/(3)`
Slope of `DA =(-1-2)/(-2-1)=1`
`:'` Slope of `AB=` Slope of `CD`
`:. AB || CD`
`:'` Slope of `BC=`Slope of `DA`
`:. BC||DA`
Now, `AB||CD` and `BC||DA`
Therefore, `ABCD` is a parallelogram.