令a=√(x-5)
7<=x<=10
2<=x<=5
所以√2<=a<=√5
a²=x-5
x=a²+5
所以圆上=2(a²-5)-a
=2a²-a-10
=2(a²-a/2+1/16-1/16)-10
=2(a-1/4)²-81/16
√2<=a<=√5
在对称轴a=1/4右边,递增
a=√2,y=-√2-6
a=√5,y=-√5
所以值域[-√2-6,-√5]