Step 1. Find the coordinates of point D.
In this problem, we have a segment called CD with two endpoints. We know one of the endpoints:
[tex](2,-1)[/tex]
And we don't know the other endpoint, but we know the midpoint:
[tex](8,3)[/tex]
We will label these known points as the first point (x1,y1) and the midpoint (xm, ym) as follows:
[tex]\begin{gathered} x_1=2 \\ y_1=-1 \\ x_m=8 \\ y_m=3 \end{gathered}[/tex]
To find the second endpoint which we will call the second point (x2,y2) we use the midpoint formulas:
[tex]\begin{gathered} x_m=\frac{x_1+x_2}{2} \\ y_m=\frac{y_2+y_2}{2} \end{gathered}[/tex]
Solving each equation respectively for x2 and y2:
[tex]\begin{gathered} x_2=2x_m-x_1 \\ y_2=2y_m-y_1 \end{gathered}[/tex]
And substituting the known values for the first point and the midpoint:
[tex]\begin{gathered} x_2=2(8)-2=16-2=14 \\ y_2=2(3)-(-1)=6+1=7 \end{gathered}[/tex]
We have found the second endpoint (x2,y2):
[tex](14,7)[/tex]
Step 2. Once we know the two endpoints of the segment CD:
[tex]\begin{gathered} (2,-1) \\ \text{and} \\ (14,7) \end{gathered}[/tex]
We make a graph for reference:
Note: the diagram is not to scale.
The length of the red line is what we are asked to find.
To find this length, draw a triangle between the points, shown here in green:
The triangle is a right triangle, this means we can use the Pythagorean theorem:
The Pythagorean theorem helps us find the hypotenuse ''x'' of the triangle when we know the legs a and b.
In this case, a and b are:
Substituting in the Pythagorean theorem:
[tex]\begin{gathered} x=\sqrt[\square]{a^2+b^2} \\ x=\sqrt[]{12^2+8^2} \end{gathered}[/tex]
Solving the operations:
[tex]\begin{gathered} x=\sqrt[]{144-64} \\ x=\sqrt[]{80} \\ x=8.9 \end{gathered}[/tex]
The solution is b. 8.9 units.
Answer: 8.9 units
