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Quadratic Crossed 2 Distinct Real Numbers (Posted on 2024-07-23) Difficulty: 2 of 5
Determine two distinct real numbers a and b that satisfy this system of equations:
a2-b = 73
b2-a =73

No Solution Yet Submitted by K Sengupta    
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Solution Solution | Comment 2 of 3 |
Both equations have the same constant.  In fact I have seen several versions of this problem differing only in the common constant term posed in various places. The common constant is usually of the form c^2+c+1, and for this problem c=8.

So likely the intended trick is to subtract the two equations, eliminating the constant and leaving behind
a^2-b^2+a-b = 0
Then factor into
(a-b)*(a+b+1) = 0

This forms two solution sets.  a=b or a=-b-1.  The first is to be discarded by the problem statement.  So I will continue with the second and use the generalized constant.
Then b^2-(-b-1) = c^2+c+1

There is the easy solution of b=c, which makes a=-c-1. But going through the quadratic formula or factoring gets a second solution of b=-c-1, which makes a=-c-1.
These two solutions are the same pair just in the opposite order which makes sense from the symmetry in the original equation.

For this specific problem c=8.  Then the two distinct real numbers a and b that satisfy the system of equations are a=8 and b=-8-1=-9, or the converse a=-9 and b=8.

  Posted by Brian Smith on 2024-07-23 10:12:20
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