搜狗做题网1,5,7,9,11,13,15.三个数相加等于30,数字可重复使用
来源：学生作业帮助网 编辑：作业帮 时间：2024/06/17 06:14:47
假设a=1/5+1/7+1/9+1/11,b=1/7+1/9+1/11原式=a*(b+1/13)-(a+1/13)*b=ab+a*1/13-ab-b*1/13=1/13*(a-b)=1/65
1+3+5+7+9+11+13+15+…+99=（1＋99）×50÷2=2500
1+3+5+7+9+11+13+.+999=(1+999)×1000÷2÷2=250000
2/3*5+1/5*7+1/7*9+1/9*11+1/11*13=1/15+1/2(1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11+1/11-1/13)=1/15+1/2(1/3-1
97,-99
原式=（1-1/3）×1/2+（1/3-1/5）×1/2+（1/5-1/7）×1/2……+（1/19-1/21）×1/2=（1-1/3+1/3-1/5+1/5-……+1/19-1/21）×1/2=（1
应该是1/(1*3)+1/(3*5)+1/(5*7)+1/(7*9)+1/(9*11)+1/(11*13)吧1/(1*3)=(1/1-1/3)/21/(3*5)=(1/3-1/5)/2.1/(11*1
提取=(1/5+1/7+1/9+1/11)*(1/7+1/9+1/11)+(1/5+1/7+1/9+1/11)*1/13-(1/5+1/7+1/9+1/11)*(1/7+1/9+1/11)-1/13*
=2433用计器算的
1/5*7+1/7*9+1/9*11+1/11*13+.+1/23*25=1/2[(1/5-1/7)+(1/7-1/9)+…+（1/23-1/25）]=1/2(1/5-1/7+1/7-1/9+……+1
设u=5/1+7/1+9/1+11/1,v=7/1+9/1+11/1,原式=u（v+13/1)-(u+13/1)v=13/1(u-v)=13/1*5/1=65/1
1.1/652.50%设成本为X定价时所期望的利润=[（20%X+X）/80%-X]/X=50%3.设乙店进价是x元,甲店的进价是(1-10%)x=0.9x元(1+15%)*x-(1+20%)*0.9
（2n-1）(-1)^(n-1)(n为正整数)
1/3*5+1/5*7+1/7*9+1/9*11+1/11*13＝1/2×﹙1/3－1/5＋1/5－1/7＋1/7－1/9＋1/9－1/11＋1/11－1/13﹚＝1/2×﹙1/3－1/13﹚＝1/2
您好!分析:1/1×3×5=1/4×(1/1×3-1/3×5)1/3×5×7=1/4×(1/3×5-1/5×7)1/5×7×9=1/4×(1/5×7-1/7×9)1/7×9×11=1/4×(1/7×9
1/5*7+1/7*9+1/9*11+1/11*13=1/2（1/5-1/7+1/7-1/9+1/9-1/11+1/11-1/13）=1/2*（1/5-1/13）=4/65
原式=1/2x(1/3-1/5)+1/2x(1/5-1/7)+1/2x(1/7-1/9)+1/2x(1/9-1/11)+1/2x(1/11-1/13)=1/2x(1/3-1/5+1/5-1/7+1/7
=1/15+1/105+1/315+1/693+1/1287+1/2145=1/16/106/316/694/1288/2145=9.7316042515123674170894241897003e-
按平常那么计前两项取1/3*1/5(1+1/7)后面也是1/7*1/9(1/5+1/11)第三个是1/11*1/13(1/9+1/15)这样计吧