A parallelogram has two pairs of equal opposite sides , so the way to get its side lengths depends on what information you already know. If you know one side, the opposite side is the same length; if you know the perimeter, use P=2a+2bP=2a+2bP=2a+2b and solve for the unknown side; if you know coordinates, use the distance formula between the shape’s vertices.

Simple rule

  • Opposite sides are equal.
  • Adjacent sides are not necessarily equal.
  • So if one side is aaa, the side across from it is also aaa.

From the perimeter

If the perimeter is known, use:

P=2a+2bP=2a+2bP=2a+2b

Then rearrange to find the missing side. For example, if P=160P=160P=160 and one side is 202020, the other side is 606060.

From coordinates

If the vertices are given on a graph, find a side length with the distance formula:

d=(x2−x1)2+(y2−y1)2d=\sqrt{(x_2-x_1)^2+(y_2-y_1)^2}d=(x2​−x1​)2+(y2​−y1​)2​

Do that for one side, then use the opposite-side rule to get the matching side. This is common in algebra and geometry problems.

Example

If a parallelogram has one side of 8 cm, the opposite side is also 8 cm. If the other side is 5 cm, then the full set of side lengths is 8 cm, 5 cm, 8 cm, 5 cm.

TL;DR

To get the sides of a parallelogram, use the fact that opposite sides are equal ; if one side is unknown, solve it from the perimeter, coordinates, or another given expression.