SOLUTION: A rhombus has half the area of the square with the same side-length. What is the ratio of the long to short diagonal?

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Question 1127404: A rhombus has half the area of the square with the same side-length. What is the ratio of the long to short diagonal?
Found 2 solutions by MathLover1, ikleyn:
Answer by MathLover1(20850) About Me  (Show Source):
You can put this solution on YOUR website!

To find the area of a rhombus use:
A=%281%2F2%29d%5B1%5D%2Ad%5B2%5D
if the sides of a rhombus is x, the area of a square is A=x%5E2
if the area of a rhombus equal to 1%2F2 the area of a square, we have
%28%281%2F2%29d%5B1%5D%2Ad%5B2%5D%29%2F2=x%5E2
%281%2F4%29d%5B1%5D%2Ad%5B2%5D=x%5E2
d%5B1%5D%2Ad%5B2%5D=4x%5E2

Using Pythagoras: triangle formed bu side and half of diagonals
%28d%5B1%5D%2F2%29%5E2%2B%28d%5B2%5D%2F2%29%5E2=x%5E2
d%5B1%5D%5E2%2F4%2Bd%5B2%5D%5E2%2F4=x%5E2
d%5B1%5D%5E2%2Bd%5B2%5D%5E2=4x%5E2.......(2)
Hence the direct relation between diagonals are not possible.
Properties of Rhombus

Answer by ikleyn(52852) About Me  (Show Source):
You can put this solution on YOUR website!
.
Let x be the common length of the square and the rhombus.


Use the formula for the rhombus area 

    Rhombus area = x%2Ax%2Asin%28alpha%29 = x%5E2%2Asin%28alpha%29,

where alpha is the angle between (any) two adjacent sides of the rhombus.



Comparing with the formula for the square area, you can conclude that sin%28alpha%29 = 1%2F2;

hence,  the angle   alpha  is either  30° or 150°, which geometrically represent the same rhombus.



Then the longer diagonal of the rhombus is  ( the law of cosine with cos(150°) = -sqrt%283%29%2F2 )

    sqrt%28x%5E2+%2B+x%5E2+-+2%2Ax%2Ax%2Acos%28150%5Eo%29%29 = sqrt%282x%5E2+-+2x%5E2%2A%28-%28sqrt%283%29%2F2%29%29%29 = x%2Asqrt%282%2B2%2A%28sqrt%283%29%2F2%29%29 = x%2Asqrt%282+%2B+sqrt%283%29%29.


The shorter diagonal is ( the law of cosine with cos(30°) = sqrt%283%29%2F2 )

    sqrt%28x%5E2+%2B+x%5E2+-+2%2Ax%2Ax%2Acos%2830%5Eo%29%29 = sqrt%282x%5E2+-+2x%5E2%2A%28sqrt%283%29%2F2%29%29 = x%2Asqrt%282-2%2A%28sqrt%283%29%2F2%29%29 = x%2Asqrt%282+-+sqrt%283%29%29.



Thus the ratio of the longer diagonal length to the shorter diagonal length is  sqrt%282%2Bsqrt%283%29%29%2Fsqrt%282-sqrt%283%29%29.


You can rationalize this fraction further

    sqrt%282%2Bsqrt%283%29%29%2Fsqrt%282-sqrt%283%29%29 = sqrt%282%2Bsqrt%283%29%29%2Fsqrt%282-sqrt%283%29%29.sqrt%282%2Bsqrt%283%29%29%2Fsqrt%282%2Bsqrt%283%29%29 = sqrt%28%282%2Bsqrt%283%29%29%5E2%29%2Fsqrt%282%5E2+-+%28sqrt%283%29%29%5E2%29 = %282%2Bsqrt%283%29%29%2F%284-3%29 = 2%2Bsqrt%283%29 = 3.732 (approximately).

Solved.


Nice solution to a nice problem.