Which is the equation of the line passing through (0, 4) and (8, 0)?
1.y= -1/2x + 4
2.y= -1/2x + 8
3.y=-2x + 4
4.y=-2x + 9

Question

Which is the equation of the line passing through (0, 4) and (8, 0)? 
1.y= -1/2x + 4
2.y= -1/2x + 8
3.y=-2x + 4
4.y=-2x + 9

e.mccormick · Answer

Know how to find the slope using any two points?

anonymous · Answer

well the slopes for 3. and 4.  would be -2/1 right?

e.mccormick · Answer

Yes.  But I am saying, start with  (0, 4) and (8, 0) and find the slope.  Know how to do that?

e.mccormick · Answer

$\dfrac{\Delta y}{\Delta x}$ AKA: $\dfrac{y_2-y_1}{x_2-x_1}$

anonymous · Answer

so 4/0 and 0/8 right?

e.mccormick · Answer

No.  

If (0, 4) is $(x_1, y_1)$ and (8, 0) is $(x_2, y_2)$  what do you get when you substitude them into $\dfrac{y_2-y_1}{x_2-x_1}$?

Ypu can copy and paste `$\dfrac{y_2-y_1}{x_2-x_1}$` if you want it formatted nice like what I type up.  Or use (y_2-y_1)/(x_2-x_1) and replace them with the right numbers.  Then you simplify.

anonymous · Answer

(0-4)/(8-0)

anonymous · Answer

-4/8? or 4/8

e.mccormick · Answer

Right.  Which becomes -4/8 and simplifies a bit more than that.

anonymous · Answer

-1/2

anonymous · Answer

so its either 1 or 2 now

e.mccormick · Answer

Yes.  So the slope, which is sometimes called m, is -1/2.  OK, so next, you need to find the intercept.

$y=mx+b$  You know m, you need b.

You have been given two points and found m.  With that, do you think you can find b?

anonymous · Answer

i forgot how to find b again haha

e.mccormick · Answer

Pick one of the two points.  Use the x and y from that point, as well as the m you just found, and plug them into $y=mx+b$.

See, $y=mx+b$ has 4 variables.  But you have 3 of the 4.  Suddently you are able to solve for the remainging 1 when you plug in the other 3.

e.mccormick · Answer

This is very typical in math.  You use what is known to find what is unknown.

You were given two points.  You need $y=mx+b$ form, so you need m and b.  The two points let you find m.  Then one point and m let you find b.  Once you know m and b, then you can see the proper $y=mx+b$ form that answers this question.

It is just a set of steps.  Each one is a small step.  They build up as you do them.  Eventually they make a solution.

anonymous · Answer

hm

anonymous · Answer

y=-1/2x +b im still confused on how to find b

e.mccormick · Answer

Personally I like the first point because x is easy in that one.

anonymous · Answer

so its y=-1/2x +4

e.mccormick · Answer

Yep!