选择题
7.设t＞0．则当t→0时，f(t)=
【正确答案】
C
【答案解析】f(t)=
=∫02πdθ∫0t(1一cosr2dr=π∫0t(1一cosr2)d(r2)=π
(1一cosr)dr．因为
=∫02πdθ∫0t(1一cosr2dr=π∫0t(1一cosr2)d(r2)=π(1一cosr)dr．因为