y=2sinX+cosXy=√5[(2√5/5)sinX+(√5/5)cosX]设存在角度a,且sina=√5/5；cosa=2√5/5角a满足sin²a+cos²a=1所以：y=√5(sinXcosa+cosXsina)=√5sin(X+a)因为-1≤sin(X+a)≤1所以：-√5≤y≤√5y=2sinX+cos...