Can you help me solve this Calculus / Vectors problem...?

x/a=y/b=z/c=1. Prove.

Can you help me solve this Calculus / Vectors problem...?
x/a=y/b=z/c=1 is wrong!


the question should be:


prove


x/a + y/b + z/c = 1





PROOF:


Assume the equation of the plane is


Ax + By + Cz = D (1)


The point (a, 0, 0) is on the plane, so Aa = D, A = D/a (2)


The point (0, b, 0) is on the plane, so


Bb = D, B = D/b (3)


The point (0, 0, c) is on the plane, so


Cc = D, C = D/c (4)


Plug (2), (3), (4) into (1) and get


x/a + y/b + z/c = 1