已知a=-2/[(根号7)+3],=-2(3-√7)/[(3-√7)(3+√7)]=-2(3-√7)/(9-7)=-(3-√7)=√7-3;代数式a^3+5a^2-4a-6的值=a^2(a+5)-4a-6=(7+9-6√7)(√7+2)-4√7+12-6=16√7+32-42-12√7+6-4√7=-4