当sinx>=cosx时，f(x)=sinx+cosx+sinx-cosx=2sinx在[π/4, 5π/4]都满足，此时(x)值域为[-√2, 2]当sinx<=cosx时， f(x)=sinx+cosx+cosx-sinx=2cosx在[-3π/4, π/4]都满足，此时f(x)值域为[-√2, 2]故f(x)的值域为[-√2, 2]
