求椭圆的焦半径公式推导

2个回答

  • 证明:

    |PF1|²

    =(x - c)² + y²

    =[a²(x - c)² + a²y²]/a²

    =[a²x² - 2a²cx + a²c² + a²y²]/a² /***--根据b²x² + a²y² = a²b² ***/

    =[a²x² - 2a²cx + a²c² + a²b² - b²x²]/a²

    =[(a²-b²)x² = 2a²cx + a²(b² + c²)]/a²

    =[c²x² -2a²cx + a^4]/a²

    =(a² - cx)²/a²

    ∴PF1 = (a² - cx)/a = a - (c/a)x = a - ex

    同理可证:PF2 = a + ex