라그랑이야ㅎㅎ

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문제

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포물선의 궤도방정식을 구하면

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이때 발사각 θ가 존재하면 (tan θ가 실근을 갖는다면)
포물선이 도달할 수 있단 뜻이므로 판별식을 쓰면

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*응용

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r = d, z = 3d/4 넣고 이차방정식을 풀면
v² = 2dg
따라서 답은 3번이다