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Straight line help

Hi guys;

Working through some Higher Past Papers from SQA, specifically 2009 paper.

Triangle PQR has vertex P on the x-axis.
Q and R are the points (4,6) and (8, –2) respectively.
The equation of PQ is 6x- 7 y + 18 = 0.

State the co-ordinates of P.

I got (-3.0).

I thought if y=0; then I would remove it; and focus solely on X; making the equation:
6x = -18
x = -3

1) Is that the correct method?
2) How do I know when it is appropiate to use the subsitution of "y=0"? Is there a name for that principle?

Reply 1

Gmart

Original post by apronedsamurai

Hi guys;

Working through some Higher Past Papers from SQA, specifically 2009 paper.

I got (-3.0).

I thought if y=0; then I would remove it; and focus solely on X; making the equation:
6x = -18
x = -3

1) Is that the correct method?
2) How do I know when it is appropiate to use the subsitution of "y=0"? Is there a name for that principle?

You know that y=0 when the question says the point is on the x-axis, as it does - your method is great, well done :smile:

Reply 2

Spandy

Yes, this is the correct method, infact the only method. Put y=0 only when you have to find a general point on the X-axis (for instance, you would've put y=2 if you had to find a point 2 units above the x-axis) And no, this is a general substitution and has no name.
Rgds

(edited 9 years ago)

Reply 3

User947387

You know that y=0 when the question says the point is on the x-axis, as it does

Sorry to sound dim, but is that because it says "vertex P on the x-axis"?

Put y=0 only when you have to find a general point on the X-axis (for instance, you would've put y=2 if you had to find a point 2 units above the x-axis)

I am a little unsure as to what you mean there. Could you please give me an example to illustriate?

(Sorry guys, maths is really a weak point for me, so want to drill down the details).

Reply 4

davros

Original post by apronedsamurai

Sorry to sound dim, but is that because it says "vertex P on the x-axis"?

.

"vertex P on the x axis" means precisely that the point P has y coordinate 0.

Reply 5

Spandy

I meant that if you have to find a point some 'p' units above x-axis, put y=p. Similarly, if you are asked to find a point 'q' units to the right of the y-axis, put x=q...