Thread: Develop algebraically an equation...

that related the height of students arm span to their height. I have collected the arm span and height in cm of 15 students in my class and have graphed them in a scatterplot. What does it mean to develop an algebraic equation relating arm span to height? How do I go about this? Is it like y=ax+b??? I have no idea! Please help

Originally Posted by bemypenguinxx

that related the height of students arm span to their height. I have collected the arm span and height in cm of 15 students in my class and have graphed them in a scatterplot. What does it mean to develop an algebraic equation relating arm span to height? How do I go about this? Is it like y=ax+b??? I have no idea! Please help

I believe that you have a positive correlation. Now, draw a line of best fit and find your equation of the line.


It will be in the form, , where:

• and will be your changing variable.
• is your gradient.
• is your -intercept.

When you have drawn your line of best fit, draw a triangle and find the gradient which is calculated by:
where are coordinates of the triangle.

will be the value at which the line of best fit crosses the -axis.

How do I know where to draw the triangle to find the gradient? I don't know where on the line of best fit to draw it????

Originally Posted by bemypenguinxx

How do I know where to draw the triangle to find the gradient? I don't know where on the line of best fit to draw it????

It doesn't matter as long as the triangle touches the line of best fit. Be sure to draw a large triangle to get an accurate value of the gradient.

OK, I think I've gotten the gradient by using that rule. I have it so y2=190, y1=140, x2=173 and x1=159.5

I did this, but I don't think it's right :S

m (gradient)
= y2-y1
x2-x1
= 190-140
173-159.5
≈ 3.57

c (y-intercept)
Using y-y1=m(x-x1)
y-140=3.57(x-159.5)
y-140=3.57x-569.42
+140 +140
y=3.57x-429.42

I then need to use that equation (presuming it's right) to predict my own arm span. Then I have to say whether it was accurate. I then need to figure out if that model is a direct variation relationship, and I have no idea what that is

THEN it says does your model have limitations in its domain and range?


Ahhh I'm really confused this is due soon and I don't know what to do

Thanks for helping.

Originally Posted by bemypenguinxx

OK, I think I've gotten the gradient by using that rule. I have it so y2=190, y1=140, x2=173 and x1=159.5

I did this, but I don't think it's right :S

m (gradient)
= y2-y1
x2-x1
= 190-140
173-159.5
≈ 3.57

c (y-intercept)
Using y-y1=m(x-x1)
y-140=3.57(x-159.5)
y-140=3.57x-569.42
+140 +140
y=3.57x-429.42

My answer came slightly different to yours. I got:

OK that makes much more sense to me now. I've worked through the equation you gave me and I understand it a lot better now. The question asks me if the model represents a direct variation relationship? I don't know what direct variation is?

Thank you for all your help so far, it's really helped me!

EDIT: Is it the same as direct proportion? If so, I have some work I've done on it but it's how to solve proportion questions, not how to identify them which I don't know how to do?