(x+c)²+y²
=(x+c)²+b²(a²-x²)/a²
=[a²(x+c)²+b²(a²-x²)]/a²
=(a²x²+2a²cx+a²c²+b²a²-b²x²)/a²
=[(a²-b²)x²+2a²cx+a²(c²+b²)]/a²
=(c²x²+2a²cx+a²*a²)/a²
=(cx+a²)²/a²
即 |PF|²=(cx+a²)²/a²
cx+a>0
|PF|=(cx+a²)/a=a+cx/a
(x+c)²+y²
=(x+c)²+b²(a²-x²)/a²
=[a²(x+c)²+b²(a²-x²)]/a²
=(a²x²+2a²cx+a²c²+b²a²-b²x²)/a²
=[(a²-b²)x²+2a²cx+a²(c²+b²)]/a²
=(c²x²+2a²cx+a²*a²)/a²
=(cx+a²)²/a²
即 |PF|²=(cx+a²)²/a²
cx+a>0
|PF|=(cx+a²)/a=a+cx/a