Finding angles of a parallelogram.

In summary: Then use the Law of Cosines to find the lengths of the sides.No, you can use some simple trig in one quadrant of the parallelogram to get the angle (in terms of A and diagonals d1 and d2). Remember, each quadrant has 1/4 the total area A. Then use the Law of Cosines to find the lengths of the sides.
  • #1
tiggertime
3
0

Homework Statement


This question has popped up recently and I was completely stumped. How to find the angles of a parallelogram given only area and the length of the diagonals? I'm trying to find a generic solution or formula that works for a non-rhombus.

Homework Equations

The Attempt at a Solution


[/B]
Parallelogram and Triangle Laws
Setting angle variable equal height(sin(angle variable))
Vector addition laws
 
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  • #2
The area of a parallelogram is "base times height". "Base" is the length of one side, b. The height is the length of perpendicular from a vertex to the base. Given angle [itex]\theta[/itex] the height is the "opposite side" while the length of that side, a, is the hypotenuse, h, so [itex]h= a sin(\theta)[/itex] The area is [itex]ab sin(\theta)[/itex].
 
  • #3
HallsofIvy said:
The area of a parallelogram is "base times height". "Base" is the length of one side, b. The height is the length of perpendicular from a vertex to the base. Given angle [itex]\theta[/itex] the height is the "opposite side" while the length of that side, a, is the hypotenuse, h, so [itex]h= a sin(\theta)[/itex] The area is [itex]ab sin(\theta)[/itex].

I still don't understand how to obtain sides a and b from the diagonal lengths since there are no right angles to use Pythagoras.
 
  • #4
I got a solution using the Law of Cosines with the angle of intersection of the diagonals to get the lengths of the sides.
 
  • #5
insightful said:
I got a solution using the Law of Cosines with the angle of intersection of the diagonals to get the lengths of the sides.

So you were given the angle of intersection? How do you find the sides without that information? I tried substituting A=absinθ into the equation, after working through sine changes, but the solution keeps cancelling out.
 
  • #6
tiggertime said:
So you were given the angle of intersection?
No, you can use some simple trig in one quadrant of the parallelogram to get the angle (in terms of A and diagonals d1 and d2). Remember, each quadrant has 1/4 the total area A.
 
Last edited:

1. What is a parallelogram?

A parallelogram is a quadrilateral with two pairs of parallel sides. This means that the opposite sides are equal in length and parallel to each other.

2. How do you find the angles of a parallelogram?

To find the angles of a parallelogram, you can use the formula A = 180 - 2B, where A represents the measure of one angle and B represents the measure of the adjacent angle.

3. Can you find the angles of a parallelogram if you only know the length of one side?

Yes, you can find the angles of a parallelogram if you know the length of one side. You can use the formula A = 180 - 2B, where A represents the measure of one angle and B represents the measure of the adjacent angle. You will need to know the length of at least one side and one angle in order to solve for the other angle.

4. Is it possible to have a parallelogram with all equal angles?

No, it is not possible to have a parallelogram with all equal angles. A parallelogram must have two pairs of opposite angles that are equal, but the other two angles will always be different.

5. Can you use the Pythagorean Theorem to find the angles of a parallelogram?

No, the Pythagorean Theorem cannot be used to find the angles of a parallelogram. This theorem only applies to right triangles, which a parallelogram is not.

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