已知非零实数a,b,c成等差数列,且公差d≠0,求证:1/(√b+√c),1/(√c+√a),1/(√a+√b)也成等差

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  • 假设d>0,则c>b>a,1/(√b+√c)=(√c-√b)/(√b+√c)*(√c-√b)=(√c-√b)/(c-b)=(√c-√b)/d,同理:1/(√c+√a)=(√c-√a)/2d;1/(√a+√b)=(√b-√a)/d.令他们分别相减,得(√c-√a)/2d-(√c-√b)/d=(2√b-√a-√c)/2d,(√b-√a)/d-(√c-√a)/2d=(2√b-√a-√c)/2d,即(√c-√a)/2d-(√c-√b)/d=(√b-√a)/d-(√c-√a)/2d,即1/(√a+√b)-1/(√c+√a)=1/(√c+√a)-1/(√b+√c),因此:1/(√b+√c),1/(√c+√a),1/(√a+√b)也成等差数列.