2020 USOJMO Problems/Problem 4
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Let be the intersection of and and be the intersection of and . [b][color=#f00]Claim: [/color][/b] By Pascal's on , we see that the intersection of and , , and are collinear. Since , we know that as well. [b][color=#f00]Claim: [/color][/b] Note that since all cyclic trapezoids are isosceles, . Since and , we know that , from which we have that is an isosceles trapezoid and . It follows that , so is an isosceles trapezoid, from which , as desired.