https://tex.z-dn.net/?f=(n-1)(n%2B4)%3D104%5C%5C%20n%5E2%2B3n-4%3D104%5C%5C%20n%5E2%2B3n-108%3D0%5C%5C%20D%3D9%2B4*1*108%3D21%5E2%5C%5C%20n%3D%5Cfrac%7B-3%2B21%7D%7B2%7D%3D9%5C%5C
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https://tex.z-dn.net/?f=(n-1)(n%2B4)%3D104%5C%5C%20n%5E2%2B3n-4%3D104%5C%5C%20n%5E2%2B3n-108%3D0%5C%5C%20D%3D9%2B4*1*108%3D21%5E2%5C%5C%20n%3D%5Cfrac%7B-3%2B21%7D%7B2%7D%3D9%5C%5C
Объяснение: