y=lgcosx cosx>0
解得定義域 2kπ - π/2 < x < 2kπ+π/2
y=根號(hào)下sinx-cosx sinx - cosx ≥0
(根號(hào)2)sin(x- π/4)≥0
解得定義域 2kπ + π/4 ≤ x ≤ 2kπ+5π/4求下列函數(shù)的值域1）sinx+cosx;2)sin^x-cosx+1;3)y=cosx/(2cosx+1)