You use_________ to emphasize(强调)that you are referring to(指的是)one thing.
A:successful
B:personal
C:single
近几十年来,我国沙漠化土地不断扩大的主要原因是;( )
已知tanα=2,计算
① 2cos(π2+α)−cos(π−α)sin(π2−α)−3sin(π+α)
② sin3α−cosαsin3α+2cosα .