设t＞0，则当t→0时，f(t)=
【正确答案】
C
【答案解析】解析：f(t)=
[1－cos(x
2
+y
2
)]dxdy =∫
0
2π
dθ∫
0
t
r(1－cosr
2
)dr =π∫
0
t
(1－cosr
2
)d(r
2
) =π
(1－cosr)dr．
[1－cos(x
2
+y
2
)]dxdy =∫
0
2π
dθ∫
0
t
r(1－cosr
2
)dr =π∫
0
t
(1－cosr
2
)d(r
2
) =π
(1－cosr)dr．