∠MAN是定值
C(-4,0),设D(0,t)
∵两圆外切
∴圆D半径=CD-圆C半径=√(t^2+16)-2
∴M(0,t+√(t^2+16)-2),N(0,t-√(t^2+16)+2)
∴kAM=[t+√(t^2+16)-2]/2√3,kAN=[t-√(t^2+16)+2]/2√3
MAN为直线AN到AM的角
∴tanMAN=(kAM-kAN)/(1+kAM*kAN)
=[(√(t^2+16)-2)/√3] / {1+[t^2-(√(t^2+16)-2)^2]/12}
=[√(t^2+16)-2]/√3 / {1+(1/12)[t^2-t^2-20+4√(t^2+16)]}
=[√(t^2+16)-2]/√3 / {(1/3)[√(t^2+16)-2]}=√3/3
∴MAN为定值60°