Let , , be real numbers and let . Prove that at least one of the equations
has real solutions.
Solution
Let us consider the first equation:
This can be rewritten as:
Let and let be such that , .
Then:
So the equation becomes:
This equation has a real solution for if and only if .
Suppose . Then the first equation has no real solution.
Now consider the second equation:
Let , , .
Then , .
So the equation becomes:
Multiply both sides by (for ):
This is a quadratic equation in with discriminant:
Since , this quadratic equation has real solutions if and only if .
But recall that we are considering the case .
So .
Therefore,
So the discriminant is positive, and the quadratic equation has real solutions for .
Therefore, at least one of the two equations has real solutions.
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