Solve ab+cd Given a^2+b^2=c^2+d^2=1 and ac+bd=0

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In summary, a, b, c, d are real numbers such that a^2 + b^2 = c^2 + d^2 = 1 and ac + bd = 0. If a = -bd/c, then b^2 = c^2 and ab+cd=0.
  • #1
Kizaru
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Homework Statement


Suppose that a, b, c, d are real numbers such that a^2 + b^2 = c^2 + d^2 = 1 and ac + bd = 0. What is ab + cd?

Homework Equations


[tex]
a^{2} + b^{2} = c^{2} + d^{2} = 1
[/tex]
[tex]ac + bd = 0
[/tex]

The Attempt at a Solution


Clearly, ac = -bd. I know the solution is 0, but I am having trouble proving or deriving it.
A few things to note (some may be useless, but this is an involved problem and I'm getting my hands dirty):

[tex]

(a + c)^{2} + (b + d)^{2} = a^{2} + b^{2} + c^{2} + d^{2} + 2(ac + bd) = 1 + 1 = 2
[/tex]
[tex]
(a + d)(b + c) = ab + ac + bd + cd = ab + 0 + cd = ab + cd

[/tex]

And there are many other expressions like these, but not sure how to piece them together. The first equation tells me that the hypotenuse of a rectangle with sides (a+c) and (b+d) is sqrt(2).
The second equation tells me the area of a rectangle with sides (a+d)(b+c) = ab+cd, assuming that one value is < 0. I know there is something else I'm missing, just not sure what it is.

Any help would be greatly appreciated.
 
Last edited:
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  • #2


I really think there should be an elegant solution. For the time being, here's a brute force approach: You have three equations in four unknowns. You can solve for three of the variables in terms of the fourth, and use that to conclude that ab + cd = 0.

If one of the numbers is zero, it is pretty straight forward to show that ab + cd = 0. So let's assume that all variables are nonzero. Then you can use ac + bd = 0 to solve for a in terms of the other variables: a = -bd/c. Plug that into a^2 + b^2 = 1 to get d in terms of b and c. Then plug that into c^2 + d^2 = 1 and see what you get. I don't want to spoil the surprise!
 
  • #3


I tried that approach earlier and obtained
[tex]
c^{2}(2-\frac{1}{b^{2}}) = 1
[/tex]
I was unable to manipulate it any further into something meaningful.
It feels like I am missing something from this approach as well.
 
  • #4


Hmm, I will double-check my own calculation but I ended up with b^2 = c^2 and a^2 = d^2.
 
  • #5


If a = -bd/c, then

[tex]a^2 + b^2 = b^2 d^2 / c^2 + b^2 = 1[/tex]

so

[tex]d^2 = c^2 (1-b^2) /b^2[/tex]

[tex]1 = c^2 + d^2 = c^2 + c^2 (1-b^2) /b^2 = c^2 \left(1 + \frac{(1 - b^2)}{b^2} \right) = \frac{c^2}{b^2} [/tex]

Hence [tex]b^2 = c^2 [/tex].
 
  • #6


owlpride said:
If a = -bd/c, then

[tex]a^2 + b^2 = b^2 d^2 / c^2 + b^2 = 1[/tex]

so

[tex]d^2 = c^2 (1-b^2) /b^2[/tex]

[tex]1 = c^2 + d^2 = c^2 + c^2 (1-b^2) /b^2 = c^2 \left(1 + \frac{(1 - b^2)}{b^2} \right) = \frac{c^2}{b^2} [/tex]

Hence [tex]b^2 = c^2 [/tex].

OH, I see my mistake. I had
[tex]a^2 = -\left(\frac{bd}{c}\right)^2[/tex]
Which is why I had the 2 inside the parentheses.

And from there we get a^2 = d^2 as well, and since one must be negative to satisfy ac+bd= 0 blah blah ab+cd = 0.

Thank you very much!
 
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  • #7


interpret it as perpendicular vectors with norm 1:a=sin(t)
b=cos(t)

c=sin(t+pi/2)=cos(t)
d=cos(t+pi/2)=-sin(t)

ab+cd=0
 
  • #8


boboYO, that is a very interesting solution. Thank you!
 

FAQ: Solve ab+cd Given a^2+b^2=c^2+d^2=1 and ac+bd=0

What is the equation being solved?

The equation being solved is ab+cd given that a^2+b^2=c^2+d^2=1 and ac+bd=0.

What are the given conditions for the equation?

The given conditions for the equation are a^2+b^2=c^2+d^2=1 and ac+bd=0.

How do you solve the equation?

To solve the equation, you can use substitution or elimination method. By substituting the given values and simplifying the equations, you can find the values of a, b, c, and d. Alternatively, you can eliminate one of the variables by multiplying one of the equations by a constant and then adding or subtracting the equations to eliminate the variable.

What is the significance of the given conditions?

The given conditions represent a system of equations that describe a Pythagorean theorem. This means that the values of a, b, c, and d represent the sides of a right triangle with sides lengths of 1. The condition ac+bd=0 also shows that the triangle is a special type of right triangle called an orthogonal or perpendicular triangle.

Are there multiple solutions to the equation?

Yes, there are multiple solutions to the equation. Using the given conditions, you can find the values of a, b, c, and d that satisfy the equation. However, there are infinite combinations of values that can satisfy the equation, making it possible to have multiple solutions.

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