11. (40 points) Find all pairs of rational numbers (a,b), for which a+b==2+3.
A number or a short expression. Spacing, $ signs and \frac vs / are all fine.
Official solution
Answer: (0.5;1.5) and (1.5;0.5).
Solution. Squaring the equality, we get a+b+2ab=2+3. Transferring a+b to the right side and squaring again, we get 4ab=(2−a−b)2+2(2−a−b)3+ +3. In this expression, all terms except 2(2−a−b)3 are rational, and therefore this term must also be rational. This is possible only if 2−a−b=0. Then we have a+b=2,2ab=3. Such a system is not difficult to solve: b=2−a, ab=a(2−a)=43. Solving the quadratic equation, we find both pairs of solutions.
Source: NuminaMath-1.5,
licensed Apache-2.0.
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