Lesson 15
Weighted Averages
- Let’s split segments using averages and ratios.
15.1: Part Way: Points
For the questions in this activity, use the coordinate grid if it is helpful to you.
- What is the midpoint of the segment connecting (1,2) and (5,2)?
- What is the midpoint of the segment connecting (5,2) and (5,10)?
- What is the midpoint of the segment connecting (1,2) and (5,10)?
15.2: Part Way: Segment
Point A has coordinates (2,4). Point B has coordinates (8,1).
- Find the point that partitions segment AB in a 2:1 ratio.
- Calculate C=\frac 13 A + \frac 23 B.
- What do you notice about your answers to the first 2 questions?
- For 2 new points K and L, write an expression for the point that partitions segment KL in a 3:1 ratio.
Consider the general quadrilateral QRST with Q=(0,0),R=(a,b),S=(c,d), and T=(e,f).
- Find the midpoints of each side of this quadrilateral.
- Show that if these midpoints are connected consecutively, the new quadrilateral formed is a parallelogram.
15.3: Part Way: Quadrilateral
Here is quadrilateral ABCD.
- Find the point that partitions segment AB in a 1:4 ratio. Label it B’.
- Find the point that partitions segment AD in a 1:4 ratio. Label it D’.
- Find the point that partitions segment AC in a 1:4 ratio. Label it C’.
- Is AB’C’D’ a dilation of ABCD? Justify your answer.
Summary
To find the midpoint of a line segment, we can average the coordinates of the endpoints. For example, to find the midpoint of the segment from A=(0,4) to B=(6,7), average the coordinates of A and B: \left(\frac{0 + 6}{2}, \frac{4+7}{2}\right) = (3,5.5). Another way to write what we just did is \frac12 (A+B) or \frac12 A + \frac12 B.
Now, let’s find the point that is \frac23 of the way from A to B. In other words, we’ll find point C so that segments AC and CB are in a 2:1 ratio.
In the horizontal direction, segment AB stretches from x=0 to x=6. The distance from 0 to 6 is 6 units, so we calculate \frac23 of 6 to get 4. Point C will be 4 horizontal units away from A, which means an x-coordinate of 4.
In the vertical direction, segment AB stretches from y=4 to y=7. The distance from 4 to 7 is 3 units, so we can calculate \frac23 of 3 to get 2. Point C must be 2 vertical units away from A, which means a y-coordinate of 6.
It is possible to do this all at once by saying C = \frac13 A + \frac23 B. This is called a weighted average. Instead of finding the point in the middle, we want to find a point closer to B than to A. So we give point B more weight—it has a coefficient of \frac23 rather than \frac12 as in the midpoint calculation. To calculate C = \frac13 A + \frac23 B, substitute and evaluate.
\frac13 A + \frac23 B
\frac13 (0,4) + \frac23 (6,7)
\left(0,\frac43 \right) + \left(4, \frac{14}{3} \right)
(4,6)
Either way, we found that the coordinates of C are (4,6).
Glossary Entries
- opposite
Two numbers are opposites of each other if they are the same distance from 0 on the number line, but on opposite sides.
The opposite of 3 is -3 and the opposite of -5 is 5.
- point-slope form
The form of an equation for a line with slope m through the point (h,k). Point-slope form is usually written as y-k = m(x-h). It can also be written as y = k + m(x-h).
- reciprocal
If p is a rational number that is not zero, then the reciprocal of p is the number \frac{1}{p}.