人教版六年级数学上册第四单元第八课时_确定起跑线 (2).ppt

复习回顾：,100米决赛运动员起跑情形,,400米决赛运动员起跑情形,,,,,为什么运动员站在不同的起跑线？,确定起跑线,小组合作：,观察每一条跑道的长度相等吗？ 怎样找出相邻两个跑道的差距？,,,,,,,,,,,,,,,,,,,,,1,,3,,2,,终点线,,,,,,,,,,,,,,,,,,,,,,,,,,85.96m,,,72.6m,,1.25m,,1.25m,,1.25m,,,,,,,,,,,,,,,,,,,,,,,,,,,85.96m,,,72.6m,,,1.25m,1.25m,1.25m,,,,每一条跑道的长度＝两个直道的长度＋圆的周长,,（长度相等）,1、跑道由（ ）和（ ）组成 2、左右两个半圆形的弯道合起来是( ) 3、现在每一圈跑道的长度可以看成 ( )两条直段跑道,,一个圆,两个直道的长度加上一个圆的周长,两个半圆形跑道,相邻两跑道的长度差 = 外跑道圆周长 - 相邻里跑道圆周长,相邻两跑道的长度差也就是相邻两个跑道的起跑线的差,注：π取3.14159,,,,,,,,,,,,,,,,,,,,,,,,,85.96m,,,72.6m,,,,,第一道,直径：,周长：,全长：,72.6m,72.6×3.14159≈228.08（m）,85.96×2＋228.08=400（m）,72.6,228.08,400,注：π取3.14159,,,,,,,,,,,,,,,,,,,,,,,,,85.96m,,,72.6m,,,,,第一道,直径：,周长：,全长：,72.6m,228.08m,400m,第二道,直径：,周长：,全长：,72.6＋1.25＋1.25=75.1（m）,75.1×3.14159≈235.93（m）,85.96×2＋235.93=407.85（m）,,,1.25m,1.25m,72.6,228.08,400,75.1,235.93,407.85,注：π取3.14159,,,,,,,,,,,,,,,,,,,,,,,,,85.96m,,,72.6m,,,,,第二道,直径：,周长：,全长：,75.1m,235.93m,407.85m,第三道,直径：,周长：,全长：,75.1＋1.25＋1.25=77.6（m）,77.6×3.14159≈243.79（m）,85.96×2＋243.79=415.71（m）,,,1.25m,1.25m,,75.1m,72.6,228.08,400,75.1,235.93,407.85,77.6,243.79,415.71,注：π取3.14159,,,,,,,,,,,,,,,,,,,,,,,,,85.96m,,,72.6m,,,,,第三道,直径：,周长：,全长：,77.6m,243.79m,415.71m,第四道,直径：,周长：,全长：,77.6＋1.25＋1.25=80.1（m）,80.1×3.14159≈251.64（m）,85.96×2＋251.64=423.56（m）,,,1.25m,1.25m,,77.6m,72.6,228.08,400,75.1,235.93,407.85,77.6,243.79,415.71,80.1,251.64,423.56,注：π取3.14159,,,,,,,,,,,,,,,,,,,,,,,,,85.96m,,,72.6m,第四道,直径：,周长：,全长：,80.1m,251.64m,423.56m,第五道,直径：,周长：,全长：,80.1＋1.25＋1.25=82.6（m）,82.6×3.14159≈259.50（m）,85.96×2＋259.5=431.42（m）,72.6,228.08,400,75.1,235.93,407.85,77.6,243.79,415.71,80.1,251.64,423.56,82.6,259.5,431.42,注：π取3.14159,,,,,,,,,,,,,,,,,,,,,,,,,85.96m,,,72.6m,第五道,直径：,周长：,全长：,82.6m,259.5m,431.42m,第六道,直径：,周长：,全长：,82.6＋1.25＋1.25=85.1（m）,85.1×3.14159≈267.35（m）,85.96×2＋267.35=439.27（m）,72.6,228.08,400,75.1,235.93,407.85,77.6,243.79,415.71,80.1,251.64,423.56,82.6,259.5,431.42,85.1,267.35,439.27,注：π取3.14159,,,,,,,,,,,,,,,,,,,,,,,,,85.96m,,,72.6m,第六道,直径：,周长：,全长：,85.1m,267.35m,439.27m,第七道,直径：,周长：,全长：,85.1＋1.25＋1.25=87.6（m）,87.6×3.14159≈275.20（m）,85.96×2＋275.2=447.12（m）,72.6,228.08,400,75.1,235.93,407.85,77.6,243.79,415.71,80.1,251.64,423.56,82.6,259.5,431.42,85.1,267.35,439.27,87.6,275.2,447.12,注：π取3.14159,,,,,,,,,,,,,,,,,,,,,,,,,85.96m,,,72.6m,第七道,直径：,周长：,全长：,87.6m,275.2m,447.12m,第八道,直径：,周长：,全长：,87.6＋1.25＋1.25=90.1（m）,90.1×3.14159≈283.06（m）,85.96×2＋283.06=454.98（m）,72.6,228.08,400,75.1,235.93,407.85,77.6,243.79,415.71,80.1,251.64,423.56,82.6,259.5,431.42,85.1,267.35,439.27,87.6,275.2,447.12,90.1,283.06,454.98,注：π取3.14159,各跑道的起跑线应该相差多少米？,7.85m,/ 7.86m,7.85,,7.86,,7.85,,7.86,,7.85,,7.85,,7.86,,,72.6,228.08,400,75.1,235.93,407.85,77.6,243.79,415.71,80.1,251.64,423.56,82.6,259.5,431.42,85.1,267.35,439.27,87.6,275.2,447.12,90.1,283.06,454.98,注：π取3.14159,我不用算出每条跑道的长度，也知道它们相差多少米?,两条直道是一样的，所以只需要看圆的周长。

圆的周长是这样算的：,第一道：72.6×π,第二道：(72.6＋2.5)×π = 72.6π＋2.5π,第三道：(75.1＋2.5)×π = 75.1π＋2.5π,第四道：(77.6＋2.5)×π = 77.6π＋2.5π,第五道：(80.1＋2.5)×π = 80.1π＋2.5π,第六道：(82.6＋2.5)×π = 82.6π＋2.5π,第七道：(85.1＋2.5)×π = 85.1π＋2.5π,第八道：(87.6＋2.5)×π = 87.6π＋2.5π,每一条跑道都在上一条跑道的基础上增加2.5π的长度你能算出2.5π是多少米吗？,相邻跑道起跑线的差距：跑道宽×2×π,,,,,,,,,,,,,,,,,,,,,终点,起点,200米的跑道，每条道的长度都是它们全长的一半第一条：400÷2=200（m）,第二条：407.85÷2=203.925（m）,第三条：415.71÷2=207.855（m）,第四条：423.56÷2=211.78（m）,第五条：431.42÷2=215.71（m）,每条道相差3.925m或3,39m所以起跑线应依次提前约3.925m思考：200米跑步比赛，跑道宽为1.25米，起跑线应依次提前多少米？,谢谢同学们 的坚持！！,。