应该是平方
f(x)
=2(2cos^2x-1)+(1-cos^2x)-4cosx
=3cos^2x-4cosx-1
=3[cos^2x-4/3cosx-1/3]
=3[cos^2x-4/3cosx+4/9-4/9-1/3]
=3[(cosx-2/3)^2-7/9]
=3 (cosx-2/3)^2-7/3
因为cosx∈[-1,1],
所以,当cosx=-1时,f(x)取最大值6
当cosx=2/3时,f(x)取最小值-7/3
应该是平方
f(x)
=2(2cos^2x-1)+(1-cos^2x)-4cosx
=3cos^2x-4cosx-1
=3[cos^2x-4/3cosx-1/3]
=3[cos^2x-4/3cosx+4/9-4/9-1/3]
=3[(cosx-2/3)^2-7/9]
=3 (cosx-2/3)^2-7/3
因为cosx∈[-1,1],
所以,当cosx=-1时,f(x)取最大值6
当cosx=2/3时,f(x)取最小值-7/3