By: Stephen Morris

Stephen Morris — Tue, 31 Mar 2009 01:03:52 +0000

This made me ask another question.

Given a value for the product/sum can you always come up with a pair of numbers that gives you that product/sum?

Is this pair unique?

To put it another way: given a number, k, can you always come up with a pair of numbers, a and b, such that a+b = ab = k?

Consider the expression (x-a)(x-b) = x^2 - (a+b)x + ab = x^2 - kx + k

This is zero only when x=a or x=b.

So a and b are the solutions to the equation x^2 - kx + k = 0.

We can find a and b by the standard forumala for a quadratic which gives

a, b = {k +- sqrt(k(k-4)) }/2

Now consider that k(k-4) term.

If it is negative we don't have a solution, not in real numbers anyway.

If it is zero we have a solution with a = b. This happens if k= 0 or k=4 which gives a=b=0 or a=b=2 respectively.

If it is positive we have a unique solution in the real numbers. k(k-4) is positive when k<0 or k> 4.

So we can always find a and b when k<= 0 or k>= 4.

Blimey! Enough with the quadratics already!

By: mathphan

mathphan — Fri, 27 Mar 2009 17:59:43 +0000

This can be solved algebraically:
[spoiler]Let the numbers be a and b
ab = a + b

Subtract b from both sides:
ab – b = a
Factor out b:
b(a – 1) = a
Divide both sides by a – 1:
b = a / (a – 1)
Basically for whatever number you give me, I subtract 1 from it and divide it into your original number.
Examples:
a = 3
b = 3/2
a = 7
b = 7/6
a = ?
b = ? / (? – 1)
Now looking at the domain of b / (b – 1), all real numbers are possible except b = 1.

Double-check:
1x = x + 1
x = x + 1
0 = 1 (no solution)

Answer:
You can stump me with the number 1.
[/spoiler]

Comments on: FL. Algebra on the Radio

By: Stephen Morris

By: mathphan