Question Video: Finding the Perimeter of a Quadrilateral Using the Triangle Midsegment Theorem Mathematics

Calculate the perimeter of 𝐷𝐸𝐢𝐹.


Video Transcript

Calculate the perimeter of 𝐷𝐸𝐢𝐹.

In the figure, we can observe that we have a large triangle on the outside, triangle 𝐴𝐡𝐢. And on the sides of this triangle, we have three pairs of congruent line segments marked. Firstly, line segments 𝐢𝐸 and 𝐡𝐸 are congruent. Line segments 𝐴𝐷 and 𝐡𝐷 are congruent. And line segments 𝐴𝐹 and 𝐢𝐹 are congruent. And so we could describe points 𝐷, 𝐸, and 𝐹 as midpoints of their respective sides in triangle 𝐴𝐡𝐢.

Now, given that we have to find the perimeter of 𝐷𝐸𝐢𝐹, which is the shaded polygon in the figure, then knowing that we have the midpoints of the sides of the triangle will be very useful. Because we can apply the triangle midsegment theorem, which is stated as the line segment connecting the midpoints of two sides of a triangle is parallel to the third side and is half its length.

So let’s firstly consider line segment 𝐹𝐷, which is a line connecting the midpoints of two sides of the triangle. By the theorem, we know that it must be parallel to the third side, which is the line segment 𝐢𝐡, and half the length of line segment 𝐢𝐡. Since we are given on the diagram that line segment 𝐢𝐡 has a length of 159.6 centimeters, then halving this, we can calculate that line segment 𝐹𝐷 has a length of 79.8 centimeters.

Now let’s think about the line segment 𝐷𝐸. By applying the theorem once more, we know that this line segment must be parallel to the third side 𝐴𝐢 and half its length. So we can write that 𝐷𝐸 equals one-half 𝐴𝐢. Given the measurement of 142.4 centimeters for line segment 𝐴𝐢, then halving this gives us that the line segment 𝐷𝐸 has a length of 71.2 centimeters.

Now, we can return to the fact that we need to find the perimeter of 𝐷𝐸𝐢𝐹, which is the distance around the outside edge. We have worked out the length of two of the sides of this quadrilateral, but what about the other two sides? Well, we can consider that we have worked out that 𝐷𝐸𝐢𝐹 has two pairs of opposite sides parallel. And so, by definition, 𝐷𝐸𝐢𝐹 is a parallelogram. And one of the properties of parallelograms is that opposite sides are congruent. So we know that line segment 𝐢𝐸 must also be 79.8 centimeters. And line segment 𝐢𝐹 is 71.2 centimeters.

Therefore, to find the perimeter, we add the lengths of the four sides of 79.8, 71.2, 79.8, and 71.2 centimeters, which gives us an answer for the perimeter of 𝐷𝐸𝐢𝐹 as 302 centimeters.

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