已知x2-2=0,那么x4+x2=______.
∵x2-2=0,
∴x2=2.
x4=(x2)2=4.
∴x4+x2=4+2=6.
以下哪个器官在急性风湿热时一般不受累()
A.关节
B.心脏
C.肾脏
D.皮肤
E.神经系统
—_______will your father come back?
—In a month. [ ]
A. How long
B. What time
C. How often
D. How soon