y=lgcosx cosx>0
解得定义域 2kπ - π/2 < x < 2kπ+π/2
y=根号下sinx-cosx sinx - cosx ≥0
(根号2)sin(x- π/4)≥0
解得定义域 2kπ + π/4 ≤ x ≤ 2kπ+5π/4求下列函数的值域1）sinx+cosx;2)sin^x-cosx+1;3)y=cosx/(2cosx+1)