单项选择题
下列二次型中是正定二次型的是( )
A．f
1
=(x
1
-x
2
)
2
+(x
2
-x
3
)
2
+(x
3
-x
1
)
2
B．f
2
=(x
1
+x
2
)
2
+(x
2
-x
3
)
2
+(x
3
+x
1
)
2
C．f
3
=(x
1
+x
2
)
2
+(x
2
+x
3
)
2
+(x
3
-x
4
)
2
+(x
4
-x
1
)
2
D．f
4
=(x
1
+x
2
)
2
+(x
2
+x
3
)
2
+(x
3
+x
4
)
2
+(x
4
-x
1
)
2