单复根a±ib，对应的通项是e^ax(C1cosbx+C2sinbx)k重复数根a±ib,对应的通项是e^ax[(C1+C2x+C3x²+..Ckx^(k-1))cosbx+(C1'+C2'x+..Ck'x^(k-1)sinbx]
