设f(x),g(x)在[a,b]上二阶可导,g’’(x)≠0,f(a)=f(b)=g(a)=g(b)=0.证明: 在(a,b)内,g(x)≠0;

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问题 设f(x),g(x)在[a,b]上二阶可导,g’’(x)≠0,f(a)=f(b)=g(a)=g(b)=0.证明:
在(a,b)内,g(x)≠0;

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答案设c∈(a,b),g(c)=0. 由g(a)=g(c)=g(b)=0,g(x)在[a,c],[c,b]上两次运用罗尔定理可得g’(ξ1)=g’(ξ2)=0,其中ξ1(a,c),ξ2∈(c,b),对g’(x)在[ξ1,ξ2]上运用罗尔定理,可得g’’(ξ3)=0. 因已知g’’(x)≠0,故g(c)≠0.

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