and a blue equilateral triangle moving so that it's tangent to the parabola and to the line . As the triangle is moving, its center (cyan) is following the path of a pink curve. At some moment, the sum of the coordinates of the triangle's center is equal to . At this moment the perimeter of the triangle can be expressed as where and are positive integers, and is square-free. Find .
The diagram shows a black parabola
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Let ( x , y ) be a point on the parabola. The centre of the triangle is given by ( x + 2 1 s , 3 2 y ) where s is the side of the triangle. We have y = x 2 and s = 3 2 3 y . The condition becomes x + 3 1 3 x 2 + 3 2 x 2 = 2 + 2 3 which is met when x = 3 . Then p = 3 s = 6 3 so submit the answer 6 + 3 = 9