2021 Fall AMC 10A Problems/Problem 20
Contents
Problem
How many ordered pairs of positive integers exist where both and do not have distinct, real solutions?
Solution 1 (Casework)
A quadratic equation does not have real solutions if and only if the discriminant is nonpositive. We conclude that:
- Since does not have real solutions, we have
- Since does not have real solutions, we have
Squaring the first inequality, we get Multiplying the second inequality by we get Combining these results, we get We apply casework to the value of
- If then from which
- If then from which
- If then from which
- If then from which
Together, there are ordered pairs namely and
~MRENTHUSIASM
Solution 2 (Graphing)
Similar to Solution 1, use the discriminant to get and . These can be rearranged to and . Now, we can roughly graph these two inequalities, letting one of them be the axis and the other be . The graph of solutions should be above the parabola and under its inverse, meaning we want points on the graph or in the first area enclosed by the two graphs: We are looking for lattice points (since and are positive integers), of which we can count .
~aop2014
Solution 3 (Oversimplified but Risky)
A quadratic equation has one real solution if and only if Similarly, it has imaginary solutions if and only if We proceed as following:
We want both to be value or imaginary and to be value or imaginary. is one such case since is Also, are always imaginary for both and We also have along with since the latter has one solution, while the first one is imaginary. Therefore, we have total ordered pairs of integers, which is
~Arcticturn
Video Solution by Mathematical Dexterity
https://www.youtube.com/watch?v=EkaKfkQgFbI
See Also
2021 Fall AMC 10A (Problems • Answer Key • Resources) | ||
Preceded by Problem 19 |
Followed by Problem 21 | |
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