“科”是最基本的生物分类单位.
已知函数 f(x)=x2+ax,x∈R.若 f(x+1)=f(-x),求 a 的值;当 a=2 时,求 g(x)=xf(x)的单调区间.
⑴____ A. French B. second C. fever D. then
⑵____ A. he B. she C. tell D. fever
⑶____ A. yesterday B. present C. be D. penfriend
⑷____ A.seven B.relative C.me D. dress
⑸____ A. before B. behind C.festival D. because