f（a）=6cos²a-√3sin2a
=3(1+cos2a)-√3sin2a
=3cos2a-√3sin2a+3
=2√3(√3/2cos2a-1/2sin2)+3
=2√3cos(2a+π/6)+3
根据题意
2√3cos(2a+π/6)+3=3-2√3
cos(2a+π/6)=-1
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