ããåçï¼
ããä¸è¬å°ï¼å¦æä¸ä¸ªæ°åä»ç¬¬2项起ï¼æ¯ä¸é¡¹ä¸å®çåä¸é¡¹çå·®çäºåä¸ä¸ªå¸¸æ°ï¼è¿ä¸ªæ°åå°±å«å
çå·®æ°åï¼arithmetic sequenceï¼ï¼è¿ä¸ªå¸¸æ°å«åçå·®æ°åçå
¬å·®ï¼common differenceï¼ï¼å
¬å·®é常ç¨åæ¯d表示ã
ããã缩åã
ããçå·®æ°åå¯ä»¥ç¼©å为A.P.ï¼Arithmetic Progressionï¼ã
ãããçå·®ä¸é¡¹ã
ããç±ä¸ä¸ªæ°aï¼Aï¼bç»æççå·®æ°åå¯ä»¥å ªç§°æç®åççå·®æ°åãè¿æ¶ï¼Aå«åaä¸bççå·®ä¸é¡¹ï¼arithmetic meanï¼ã
ããæå
³ç³»ï¼Aï¼ï¼aï¼bï¼/2
ãããé项å
¬å¼ã
ããan=a1+(n-1)d
ããan=Sn-S(n-1) (nâ¥2)
ãããån项åã
ããSn=n(a1+an)/2=n*a1+n(n-1)d/2
ãããæ§è´¨ã
ããä¸ä»»æ两项amï¼ançå
³ç³»ä¸ºï¼
ããan=am+(n-m)d
ããå®å¯ä»¥çä½çå·®æ°å广ä¹çé项å
¬å¼ã
ããä»çå·®æ°åçå®ä¹ãé项å
¬å¼ï¼ån项åå
¬å¼è¿å¯æ¨åºï¼
ããa1+an=a2+an-1=a3+an-2=â¦=ak+an-k+1ï¼kâ{1,2,â¦,n}
ããè¥mï¼nï¼pï¼qâN*ï¼ä¸m+n=p+qï¼åæ
ããam+an=ap+aq
ããSm-1=(2n-1)anï¼S2n+1=(2n+1)an+1
ããSkï¼S2k-Skï¼S3k-S2kï¼â¦ï¼Snk-S(n-1)kâ¦æçå·®æ°åï¼ççã
ããåï¼ï¼é¦é¡¹ï¼æ«é¡¹ï¼Ã项æ°Ã·2
ãã项æ°ï¼ï¼æ«é¡¹-é¦é¡¹ï¼Ã·å
¬å·®ï¼1
ããé¦é¡¹=2å÷项æ°-æ«é¡¹
ããæ«é¡¹=2å÷项æ°-é¦é¡¹
ãã设a1,a2,a3为çå·®æ°åãåa2为çå·®ä¸é¡¹,å2åça2çäºa1+a3ï¼å³2a2=a1+a3ã
çæ¯æ°åãããå®ä¹ã
ããä¸è¬å°ï¼å¦æä¸ä¸ªæ°åä»ç¬¬2项起ï¼æ¯ä¸é¡¹ä¸å®çåä¸é¡¹çæ¯çäºåä¸ä¸ªå¸¸æ°ï¼è¿ä¸ªæ°åå°±å«åçæ¯æ°åï¼geometric sequenceï¼ãè¿ä¸ªå¸¸æ°å«åçæ¯æ°åçå
¬æ¯ï¼common ratioï¼ï¼å
¬æ¯é常ç¨åæ¯q表示ã
ããã缩åã
ããçæ¯æ°åå¯ä»¥ç¼©å为G.P.ï¼Geometric Progressionï¼ã
ãããçæ¯ä¸é¡¹ã
ããå¦æå¨aä¸bä¸é´æå
¥ä¸ä¸ªæ°Gï¼ä½¿aï¼Gï¼bæçæ¯æ°åï¼é£ä¹Gå«åaä¸bççæ¯ä¸é¡¹ã
ããæå
³ç³»ï¼G^2ï¼abï¼Gï¼Â±(ab)^(1/2)
ãã注ï¼ä¸¤ä¸ªéé¶åå·çå®æ°ççæ¯ä¸é¡¹æ两个ï¼å®ä»¬äºä¸ºç¸åæ°ï¼æ以G^2=abæ¯a,G,bä¸æ°æçæ¯æ°åç
å¿
è¦ä¸å
åæ¡ä»¶ã
ãããé项å
¬å¼ã
ããan=a1q^(n-1)
ããan=Sn-S(n-1) (nâ¥2)
ãããån项åã
ããå½qâ 1æ¶ï¼çæ¯æ°åçån项åçå
¬å¼ä¸º
ããSn=a1(1-q^n)/(1-q)=(a1-an*q)/(1-q) (qâ 1)
ãããæ§è´¨ã
ããä»»æ两项amï¼ançå
³ç³»ä¸ºan=am·q^(n-m)
ããï¼3ï¼ä»çæ¯æ°åçå®ä¹ãé项å
¬å¼ãån项åå
¬å¼å¯ä»¥æ¨åºï¼ a1·an=a2·an-1=a3·an-2=â¦=ak·an-k+1ï¼kâ{1,2,â¦,n}
ããï¼4ï¼çæ¯ä¸é¡¹ï¼aq·ap=ar^2ï¼arå为apï¼aqçæ¯ä¸é¡¹ã
ããè®°Ïn=a1·a2â¦anï¼åæÏ2n-1=(an)2n-1ï¼Ï2n+1=(an+1)2n+1
ããå¦å¤ï¼ä¸ä¸ªå项å为æ£æ°ççæ¯æ°åå项åå
åºæ°æ°åææä¸ä¸ªçå·®æ°åï¼åä¹ï¼ä»¥ä»»ä¸ä¸ªæ£æ°C为åºï¼ç¨ä¸ä¸ªçå·®æ°åçå项åææ°æé å¹Canï¼åæ¯çæ¯æ°åãå¨è¿ä¸ªæä¹ä¸ï¼æ们说ï¼ä¸ä¸ªæ£é¡¹çæ¯æ°åä¸çå·®æ°åæ¯âåæâçã
ããæ§è´¨ï¼
ããâ è¥ mãnãpãqâN*ï¼ä¸mï¼n=pï¼qï¼åam·an=ap·aqï¼
ããâ¡å¨çæ¯æ°åä¸ï¼ä¾æ¬¡æ¯ k项ä¹åä»æçæ¯æ°å.
ããâGæ¯aãbççæ¯ä¸é¡¹ââG^2=abï¼Gâ 0ï¼â.
ãã(5) çæ¯æ°åån项ä¹åSn=A1(1-q^n)/(1-q)
ããå¨çæ¯æ°åä¸ï¼é¦é¡¹A1ä¸å
¬æ¯qé½ä¸ä¸ºé¶.
ãã注æï¼ä¸è¿°å
¬å¼ä¸A^n表示Açn次æ¹ã
ä¸è¬æ°åçé项æ±æ³ããä¸è¬æï¼
ããan=Sn-Sn-1 ï¼nâ¥2ï¼
ããç´¯åæ³ï¼an-an-1=... an-1 - an-2=... a2-a1=...å°ä»¥ä¸å项ç¸å å¯å¾anï¼ã
ããéåå
¨ä¹æ³ï¼å¯¹äºåä¸é¡¹ä¸åä¸é¡¹åä¸å«ææªç¥æ°çæ°åï¼ã
ããåå½æ³ï¼å°æ°ååå½¢ï¼ä½¿åæ°åçåæ°æä¸æåä¸å¸¸æ°çåæçå·®æçæ¯æ°åï¼ã
ããç¹å«çï¼
ããå¨çå·®æ°åä¸ï¼æ»æSn S2n-Sn S3n-S2n
ãã2(S2n-Sn)=(S3n-S2n)+Sn
ããå³ä¸è
æ¯çå·®æ°å,åæ ·å¨çæ¯æ°åä¸ãä¸è
æçæ¯æ°å
ããä¸å¨ç¹æ³ï¼å¸¸ç¨äº
åå¼çé项éæ¨å
³ç³»ï¼
æ°ååN项åå
¬å¼çæ±æ³ãã(ä¸)1.çå·®æ°å:
ããé项å
¬å¼an=a1+(n-1)d é¦é¡¹a1,å
¬å·®d, an第n项æ°
ããan=ak+(n-k)d ak为第k项æ°
ããè¥a,A,bææçå·®æ°å å A=(a+b)/2
ãã2.çå·®æ°åån项å:
ãã设çå·®æ°åçån项å为Sn
ããå³ Sn=a1+a2+...+an;
ããé£ä¹ Sn=na1+n(n-1)d/2
ãã=dn^2(å³nç2次æ¹) /2+(a1-d/2)n
ããè¿æ以ä¸çæ±åæ¹æ³: 1,ä¸å®å
¨å½çº³æ³ 2 ç´¯å æ³ 3 ååºç¸å æ³
ãã(äº)1.çæ¯æ°å:
ããé项å
¬å¼ an=a1*q^(n-1)(å³qçn-1次æ¹) a1为é¦é¡¹,an为第n项
ããan=a1*q^(n-1),am=a1*q^(m-1)
ããåan/am=q^(n-m)
ãã(1)an=am*q^(n-m)
ãã(2)a,G,b è¥ææçæ¯ä¸é¡¹,åG^2=ab (a,b,Gä¸çäº0)
ãã(3)è¥m+n=p+q å amÃan=apÃaq
ãã2.çæ¯æ°åån项å
ãã设 a1,a2,a3...anææçæ¯æ°å
ããån项åSn=a1+a2+a3...an
ããSn=a1+a1*q+a1*q^2+....a1*q^(n-2)+a1*q^(n-1)(è¿ä¸ªå
¬å¼è½ç¶æ¯æåºæ¬å
¬å¼,ä½ä¸é¨åé¢ç®ä¸æ±ån项åæ¯å¾é¾ç¨ä¸é¢é£ä¸ªå
¬å¼æ¨å¯¼ç,è¿æ¶å¯è½è¦ç´æ¥ä»åºæ¬å
¬å¼æ¨å¯¼è¿å»,æ以å¸æè¿ä¸ªå
¬å¼ä¹è¦ç解)
ããSn=a1(1-q^n)/(1-q)=(a1-an*q)/(1-q);
ãã注: qä¸çäº1;
ããSn=na1 注:q=1
ããæ±åä¸è¬æ以ä¸5个æ¹æ³: 1,å®å
¨å½çº³æ³ï¼å³
æ°å¦å½çº³æ³ï¼ 2 ç´¯ä¹æ³ 3
éä½ç¸åæ³ 4 ååºæ±åæ³ 5
è£é¡¹ç¸æ¶æ³