f（x）=cosx- 1／2cos2x的值域 - 知识问答

f（x）=cosx- 1／2cos2x的值域

1个回答

cos2x=2cos^2x-1
f(x)=cosx-1/2(2cos^2x-1)
=cosx-cos^2x+1/2
=-(cos^2x-cosx+1/4)+3/4
=-(cosx-1/2)^2+3/4
cosx=1/2 f(x)max=3/4
cosx=-1 f(x)min=-3/2

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