Given B ^ 2 + C ^ 2 = 2A ^ 2-3a + 7, BC = a ^ 2-2a-1, then the value range of a is_______________ Why? ...

∵b^2+c^2≥2bc
∴b^2+c^2=2a^2-3a+7≥2（a^2-2a-1）
If and only if B = C, the equation holds

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