Petrach is a crater on Mercury. It is ~170km in diameter. This crater is located, within the distorted terrain, on the opposite side of the planet from the Caloris Basin.
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... square trinomial of the form x2 + 2xy + y2. Since "2xy" in this case is (b/a)x, we must have y = b/(2a), so we add the square of b/(2a) to both sides, ...