21.已知雙曲線C:
=1(a>0,b>0)的左、右焦點(diǎn)分別為F
1,F(xiàn)
2,且|F
1F
2|=8,過(guò)F
2作其中一條漸近線的垂線,垂足為M,延長(zhǎng)F
2M交另一條漸近線于點(diǎn)N,且|F
2M|=|MN|,
(1)求C的方程;
(2)如圖,過(guò)A(6,0)作直線l(l不與x軸重合)與曲線C的兩支交于P,Q兩點(diǎn),直線F
1P,F(xiàn)
1Q與C的另一個(gè)交點(diǎn)分別為S,T,求證:直線ST經(jīng)過(guò)定點(diǎn).