Posted by **Sal** on Wednesday, January 30, 2013 at 6:25pm.

You are given the cordinates of the four points

A (6,2), B(2.4), C (-6, -2) and D (-2,-4).

Show that the equation of the line DA is 4y-3x = -10

help please!!

- Geometry -
**Reiny**, Wednesday, January 30, 2013 at 6:40pm
sub in the points A(6,2) and D(-2,-4)

into 4y - 3x = -10

for (6,2)

LS = 4(2) - 3(6) = -10 = RS

for (-2,-4)

LS = 4(-4) - 3(-2) = -10 = RS

YES , the equation of DA is 4y-3x = -10

Where do the other two points enter the picture ?

- Geometry -
**Sal**, Wednesday, January 30, 2013 at 6:43pm
Could you please explain that in full, as im really struggling to understand. The other two points were for calculating gradients AB, CB, DC and DA but i got that.

Thanks reiny!

- Geometry -
**Sal**, Wednesday, January 30, 2013 at 6:45pm
Is there a way of simplifying the equation 4y-3x=-10 to y=0.75x - 2.5, then proving that is the equation for DA?

- Geometry -
**Reiny**, Wednesday, January 30, 2013 at 6:54pm
I don't know how much further I can explain it.

Since both points satisfy the given equation, the equation must be the correct one

All I did was replace the x and y in the equation with the x and y of each of the points

LS stands for left side of the equation

RS stands for the right side.

in

4y - 3x = -10

4y = 3x - 10

y = (3/4)x - 10/4

or

y = .75x - 2.5

The last equation and the starting equation are really the same thing.

Subbing in either one gives you the same result

e.g.

sub in (6,2) into y = .75x - 2.5

LS = 2

RS = .75(6) - 2.5 = 4.5 - 2.5 = 2

since LS = RS , the point lies on the equation.

I will let you sub in the second point in the same way.

Let me know if you did NOT get it.

- Geometry -
**Sal**, Wednesday, January 30, 2013 at 7:16pm
What is RS and LS?

- Geometry -
**Reiny**, Wednesday, January 30, 2013 at 7:17pm
Did you not read my reply ? I explained what they are.

- Geometry -
**Sal**, Wednesday, January 30, 2013 at 7:23pm
Im sorry!!! Its so late and im half asleep. Worse time to do maths. I get it reiny!!!!!!!

Thank you

So the second point is -4 = -4

therefore it must be the correct equation as both points satisfy the equation?

- Geometry -
**Reiny**, Wednesday, January 30, 2013 at 7:32pm
You got it.

- Geometry -
**Sal**, Wednesday, January 30, 2013 at 7:37pm
Your a star!!!!!!!!!!!!!

Many thanks

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