Parametars B and D of line

Hi Physicsissuef!

Hint: you want z to be constant, but you don't want y to be constant.

So … ?

 

But how do you stop y being infinite, then?

 

I think you are putting far too much emphasis on plugging into formulas rather that thinking about what the problem says. In the xy plane, z is not just constant- it is specifically 0. If z= 0 then your equations become 3x- 2y- 9= 0 and 3x+ By+ D= 0. In the xy plane, those are each the equation of a line. For what B and D are those the same line?

 

Reread your original question! The equations represent a single line.

 

I was referring to the fact that your equation y/2 = (z - (D+27)/2)/(B + 6) seems to make y fixed and infinite if B + 6 = 0, while you want y to be able to take any value.

Well, yes, the line is parallel to a line through the origin with (a_1,a_2,0).

But you're concentrating too much on formulas, as HallsofIvy said.

btw, his method is a lot easier than the one you chose … if you do that in the exam, the examiner wil get the impression you're using the formula without really understanding it.