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# Can you calculate this? The points (-2, -4), (0,0), (4,a) and (b, 18) is on a straight line What is the number a and b?

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The points (-2, -4), (0,0), (4,a) and (b, 18) is on a straight line . What is the number a and b?

I wonder if it is possible to calculate the answer to the question, or are you just simply doing guesswork?
I don't know if I'm solving it the right way.

The answer is a = 8 and b = 9

Guest Oct 22, 2014

#3
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Thanks guys so much for the answers! People like you are gold when you're trying to learning math without a teacher!

Guest Oct 22, 2014
#1
+92808
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No..we don't usually like to do too much "guessing" on here....

Since all the points lie on a line, the slope between them will be the same. So the slope between (-2, -4) and (0, 0) = [-4 - 0]/ [-2, 0] = -4/-2 = 2

So, using this, the slope between (0, 0) and (4, a) = 2  ....and we have..

[a - 0] / [4 - 0] = 2   → a/4 = 2  → (multiply both sides by 2)......a = 8.

Similarly.......

[18 - 0 ] / [ b - 0] =  2  →  18/2 = b → 9 = b

And that's it .....(without the guesswork .....!!!)

CPhill  Oct 22, 2014
#2
+5

Points (-2, -4), (0,0), (4,a) and (b, 18)

If they're all on the same line, then they're all on the same slope, or share the same "m" value in y=mx+b. Now, in the first two ordered pairs, the x values are -2 and 0. The y values are -4 and 0. For each respective ordered pair, the equations can be made:

-4=m(-2)+b

0=m(0)+b

If you take the second equation, you can solve for b easily, because anything times 0 is 0.

0=m(0)+b

0=0+b

0=b

So, since this is all on the same line, or slope, that value of b works in the other equation.

-4=m(-2)+0

Now, solve for m! You can drop the plus 0, and then divide by -2 on both sides.

-4=m(-2)+0

-4=m(-2)

2=m

So, now we know m=2 and b=0. We can plug these into our slope-intercept equation. From now on:

y=2x+0

Or, simply,

y=2x

Now, we need to solve for (4,a) and (b, 18).

In the first one, we know the x value, 4, but need to find y, which is a.

a=2(4)

a=8

In the second one, we know the y value, 18, but need to find x, which is b.

18=2b

b=9

So you were right! a=8 and b=9.

SIDE NOTE: Because one ordered pair was (0,0), we could skip a step to find b in y=mx+b. Normally, this is how it would be done:

Find slope-intercept form of (-2,-4) and (0,0)

-4=m(-2)+b

0=m(0)+b

Rewrite both equations so b is left alone. On the first one, subtract m(-2) from both sides.

-4-m(-2)=b

This could be rewritten as -(-2m), or simply +2m.

-4+2m=b.

Now, subtract m(0) from both sides of the second equation

0-m0=b.

So, if -4+2m=b, and b=0-m0, we can squish that all together:

-4+2m=b=0-m0

-4+2m=0-m0

Then, you would go about solving for m like you would in any equation by moving all the b values to one side and all the constants (numbers without variables) to the other. We will add 0 to both sides, and subtract -2m from both sides. This will put the constants on the left and the m values on the right.

-4+0+2m-2m=0+0-m0-2m

-4=-2m

Now, dividing -4/-2 gets:

m=2

Just like it did before! Now you plug this back into the equations (the ones using points (-2,-4) and (0,0) in the slope-intercept equation y=mx+b) to get your b value. Then solve the rest like you did already!

Guest Oct 22, 2014
#3
+5

Thanks guys so much for the answers! People like you are gold when you're trying to learning math without a teacher!

Guest Oct 22, 2014
#4
+94120
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Gracious people like you make our efforts all worthwhile.  :)

Thank you :)))