Consider the area of the part of the plane z=ax+by+c that projects onto some region D in the xy-plane. If the area of the region D in the xy-plane is k, explain why the area of the part of the plane...

Consider the area of the part of the plane z=ax+by+c that projects onto some region D in the xy-plane.   If the area of the region D in the xy-plane is k, explain why the area of the part of the plane z=ax+by+c that projects onto D is
ksqrt(1+a^2+b^2)