设 f ( x ) 是可导函数, 则 lim Δ x → 0 f 2 ( x + △ x ) − f 2 ( x ) △ x = ( )。
A.
[ f ′ ( x ) ] 2 " role="presentation"> [ f ′ ( x ) ] 2
B.
2 f ′ ( x ) " role="presentation"> 2 f ′ ( x )
C.
2 f ( x ) f ′ ( x ) " role="presentation"> 2 f ( x ) f ′ ( x ) " role="presentation"> 2 f ( x ) f ′ ( x ) x ) 2 f ( x ) f ′ ( x ) " role="presentation"> f ( x ) f ′ ( x )