求函数y = (1-sinx)/(2cosx+3) 的值域
y = (1-sinx)/(2cosx+3) ≥ 0
--->1-sinx = 2ycosx+3y
--->2ycosx+sinx = 1-3y
--->√[(2y)²+1] sin(x+T) = 1-3y .tanT=2y
--->1-3y≤√[(2y)²+1]
--->1-6y+9y²≤4y²+1--->5y²-6y=y(5y-6)≤0--->0≤y≤6/5