椭圆第二焦半径公式推导
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...