抛物线y=-1/3(x-h)²+K的顶点（h,k）;
由题意知 h^2=k
y=-1/3(x-h)²+K=-1/3(x-h)²+h^2;
y=0; h^2=1/3(x-h)²; x-h=±√3h,
因为抛物线在x轴上截得的线段长是4√3,
所以x-h=2√3
则±√3h=2√3
h=正负2;k=4
恰好我做过这题...
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