Midpoint formula
The point halfway between the endpoints of a line segment is called the midpoint. A midpoint sliced a line segment into 2 equal parts.
The first calculation method:
If the line segments are vertical or horizontal, you might discover the midpoint by easily slicing the length of the segment by two and counting that value from either of the endpoints.
Discover the midpoints of line segments AB and CD.
The length of line segment A and B is 8. The midpoint is four units from either endpoint. On the graph, this point is (1 ; 4).
The length of line segment C and D is 3. The midpoint is 1.5 units from either endpoint. On the graph, this point is (2,1 : 5)
The second midpoint calculation method:
If the line segments are diagonal, more difficult must be paid to the solution. When you're finding the coordinates of the midpoint of a segment, you're actually finding the average of the X coordinates and the average of the Y coordinates.
This concept of finding the average of the coordinates could be written as a formula: 
The midpoint of a segment with endpoints (x1 , y1) and (x2 , y2) has coordinates
Other Methods of Solution:
Verbalizing the algebraic solution:
Some students like to do these "tricky" problems by just examining the coordinates and asking themselves the following questions:
1) "My midpoint's x-coordinate is -1. What is -1 half of?)
2) What do I add to my endpoint's X coordinate of +1 to get -2?)
This answer must be the X coordinate of the other endpoint."
These students are simply verbalizing the algebraic solution.
Utilizing the concept of slope and congruent triangles:
A line segment is part of a direct line whose slope remains the same no matter where it's calculated. Some students like to look at the rise and run values of the X and Y coordinates and utilize these values to find the missing endpoint. Find the slope between points C and M. This slope has a run of 2 units to the left and a rise of 4 units up. By repeating this slope from point M, you'll arrive at the other endpoint.
By using this slope approach, you're creating two congruent right triangles whose legs are the same lengths. Consequently, their hypotenuses are the same lengths too and DM = MC making M the midpoint of segment CD.
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