Question

求函数y=cosx+cos2x的值域

Answer 1

y=cosx+cos2x=cosx+2*(cosx)^2-1=2(cosx+1/4)^2-1/8-1=2(cosx+1/4)^2-9/8
∵cosx属于[-1,1]
∴y属于[-9/8,2]

望采纳！

Answer 2

解：y=cosx+2cos^2x-1
设t=cosx 则-1<=t<=1
则y=t+2t^2-1
剩下自己解决！