# Is the slope of my line wrong?

• Apr 24th 2013, 04:25 AM
alexpasty2013
Is the slope of my line wrong?
I have the co ordinates A (-2 , -2) and B (3 , 1) and I have come up with the slope of my line being -1 using the formula m=y2-y1/x2-x1 . Is this correct it feels wrong?
• Apr 24th 2013, 04:31 AM
sa-ri-ga-ma
Re: Is the slope of my line wrong?
It is wrong. Substitute the values with proper sign.
• Apr 24th 2013, 04:35 AM
emakarov
Re: Is the slope of my line wrong?
You forgot that you are subtracting negative numbers. Recall that x - (-y) = x + y.

Also, the formula for slope is (y2 - y1) / (x2 - x1). There is a big difference between this and y2 - (y1 / x2) - x1.
• Apr 24th 2013, 05:27 AM
alexpasty2013
Re: Is the slope of my line wrong?
so I have come up with -3/5 now - is this right?
• Apr 24th 2013, 05:37 AM
emakarov
Re: Is the slope of my line wrong?
Quote:

Originally Posted by alexpasty2013
so I have come up with -3/5 now - is this right?

The absolute value is correct, but the line rises from y = -2 at x = -2 to y = 1 at x = 3, so the slope must be positive.

If you'd like, post the details of your calculations for checking.
• Apr 24th 2013, 06:13 AM
alexpasty2013
Re: Is the slope of my line wrong?
I had m = 3-(-2)/1-(-2) = -3/5. Point a is (-2,-2) point b is (3,1). I also came up with a midpoint of (-0.5, 0.5) which lies on the line.
• Apr 24th 2013, 06:23 AM
emakarov
Re: Is the slope of my line wrong?
$\displaystyle 3-(-2)/1-(-2) = 3-\frac{-2}{1}-(-2)=3-(-2)-(-2)=3+2+2=7$, but this has no connection with the original problem.
• Apr 24th 2013, 06:40 AM
HallsofIvy
Re: Is the slope of my line wrong?
Quote:

Originally Posted by alexpasty2013
I had m = 3-(-2)/1-(-2) = -3/5. Point a is (-2,-2) point b is (3,1). I also came up with a midpoint of (-0.5, 0.5) which lies on the line.

If you mean (3- (-2))/(1-(-2)) then you are not doing what you were told before. 3-(-2)= 3+ 2= 5. 1-(-2)= 1+ 2= 3. The slope is 5/3. No, the midpoint is NOT "(-0.5, 0.5)" nor does that point lie on the line. You were given points (-2 , -2) and B (3 , 1). The midpoint is ((-2+3)/2, (-2+1)/2)= (1/2, -1/2) or (0.5, -0.5). I don't know whether you are confusing x and y, using an incorrect formula, or just doing the arithmetic wrong.
• Apr 24th 2013, 06:53 AM
emakarov
Re: Is the slope of my line wrong?
Quote:

Originally Posted by HallsofIvy
If you mean (3- (-2))/(1-(-2)) then you are not doing what you were told before. 3-(-2)= 3+ 2= 5. 1-(-2)= 1+ 2= 3. The slope is 5/3.

No, the slope is 3/5 because the OP has switched the numerator and denominator in post #6.
• Apr 24th 2013, 07:30 AM
alexpasty2013
Re: Is the slope of my line wrong?
the point (-0.5,0.5 does lie on the line on my graph? Apparently in the middle. For this I used the formula (1/2(x1+x2) , (1/2(11+22).

So I have 3 points, A (-2 , -2), B (3 , 1) and C (6 , 2)

I have calculated slope of line AB = 3/5 with a midpoint at (0.5 , -0.5) and slope of line CB = -1 with a midpoint of (4.5 , -0.5).

All points satisfy a graph with axis scaled at 0 - 0.5 - 1 -1.5 Etc.