MN丄AB于P，
求证：
AM²-AN²=BM²-BN2

AM²-AP² = MP²
AN²-AP² = NP²
以上两式相减得
AM²-AN² = MP²-NP²...①
同理，
BM²-BN² = MP²-NP²...②
由式①、②得
AM²-AN² = BM²-BN²