),b=(cosx,-1).(1)当a
∥b时,求2cos2x-sin2x的值;(2)求f(x)=(a
+b)·b在［
,0］上的值域. 
),b=(cosx,-1).(1)当a
∥b时,求2cos2x-sin2x的值;(2)求f(x)=(a
+b)·b在［,0］上的值域. 解
:(1)∵a∥b,∴cosx+sinx=0.∴tanx=.2cos2x-sin2x=
=
=
.
(2)∵a
+b=(sinx+cosx,
),∴f(x)=(a+b)·b=
sin(2x+
).∵-
≤x≤0,∴
≤2x+
≤
.7分∴-1≤sin(2x+
)≤
.
∴
≤f(x)≤
.11分∴函数f(x)的值域为［
,
］.