ï¼Jensenï¼ä¸çå¼ å¦æf(x)å¨(a,b)ä¸æ¯å¸å½æ°ï¼x1ï¼x2é½å¨(a,b)ä¸ï¼è¯æä¸çå¼ï¼f[(x1+x2)/2]â¥1/2[f(x1)+f(x2)]æç«.
è¯æï¼è¯æf[(x1+x2)/2]â¥1/2[f(x1)+f(x2)]æç«å¯ä»¥è½¬å为è¯æf[(x1+x2)/2]-f(x1)â¥f(x2)-f[(x1+x2)/2]æç«.ä¸å¦¨è®¾x1<x2.æ ¹æ®ææ ¼ææ¥ï¼Lagrangeï¼ä¸å¼å®çï¼å¯å¾ï¼f[(x1+x2)/2]-f(x1)=fâ(ξ1)(x2-x1)/2, f(x2)-f[(x1+x2)/2=fâ(ξ2)(x2-x1)/2,å
¶ä¸Î¾1å¨x1å(x1+x2)/2ä¹é´ï¼Î¾2å¨(x1+x2)/2åx2ä¹é´ï¼ç±åå®æ¡ä»¶x1<x2å¯ç¥ï¼Î¾1<ξ2.ç±äºf(x)å¨(a,b)ä¸æ¯å¸å½æ°ï¼æ以f(x)å¨(a,b)ä¸æ»¡è¶³fââ(x)<0,æ以fâ(x)å¨(a,b)ä¸éåï¼ç±äºÎ¾1<ξ2ï¼åæfâ(ξ1)>fâ(ξ2)ï¼æ以{f[(x1+x2)/2]-f(x1)}-{f(x2)-f[(x1+x2)/2]}=(x2-x1)[ fâ(ξ1)- fâ(ξ2)]/2>0,æ以f[(x1+x2)/2]-f(x1)>f(x2)-f[(x1+x2)/2]ï¼æ以f[(x1+x2)/2]>1/2[f(x1)+f(x2)].å¦æå设x1<x2ï¼ç»ææ¯ä¸æ ·çï¼å¦æx1=x2ï¼åæ¾ç¶f[(x1+x2)/2]=1/2[f(x1)+f(x2)],å æ¤æ们è¯æäºf[(x1+x2)/2]â¥1/2[f(x1)+f(x2)]æç«.
åçå¦æf(x)å¨(a,b)ä¸æ¯å¹å½æ°ï¼x1ï¼x2é½å¨(a,b)ä¸ï¼åæä¸çå¼ï¼1/2[f(x1)+f(x2)]â¥f[(x1+x2)/2]æç«.
对f(x)=tanxæ±äºé¶å¯¼æ°ï¼f'(x)=1/cos^2x
f''(x)=1/cos^3x*(-2)*(cosx)'=2tanx/cos^2x
æ¾ç¶å½xâ(0,Ï/2)æ¶f''(x)>0,æ¯å¹å½æ°ï¼æ
æ1/2[f(x1)+f(x2)]>f[(x1+x2)/2].
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