已知a属于R,若函数f（x）=cosa*x^2-4

已知a属于R,若函数f（x）=cosa*x^2-4sina*x+6

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f（x）=cosa*x^2-4sina*x+6
=7-sina*x^2-4sina*x
=11-(sina*x^2+4sina*x+4)
=11-(sina*x+2)^2>0
则(sina*x+2)^2