早教吧作业答案频道 -->数学-->
两等差数列{an}{bn}的钱n项和比Sn/Sn=5n+3/2n+7则a5/b5的值是若x>0y>0且2/y+8/x=1则x+y的最小值
题目详情
两等差数列{an} {bn}的钱n项和比 Sn/Sn=5n+3/2n+7 则 a5/b5的值是
若x>0 y>0 且2/y+8/x=1 则x+y的最小值
若x>0 y>0 且2/y+8/x=1 则x+y的最小值
▼优质解答
答案和解析
1.
Sn/S′n=(5n+3)/(2n+7)
a5/b5= (2a5)/(2b5)=(a1+a9)/(b1+b9)
=[9(a1+a9)/2]/[9(b1+b9)/2]= S9/S′9
=(5•9+3)/(2•9+7)
=48/25.
2.
x+y=(x+y)(2/y+8/x)
=2x/y+8+2+8y/x
=10+2x/y+8y/x≥10+2√(2x/y•8y/x)=18
2x/y=8y/x即x=2y时取到等号.
Sn/S′n=(5n+3)/(2n+7)
a5/b5= (2a5)/(2b5)=(a1+a9)/(b1+b9)
=[9(a1+a9)/2]/[9(b1+b9)/2]= S9/S′9
=(5•9+3)/(2•9+7)
=48/25.
2.
x+y=(x+y)(2/y+8/x)
=2x/y+8+2+8y/x
=10+2x/y+8y/x≥10+2√(2x/y•8y/x)=18
2x/y=8y/x即x=2y时取到等号.
看了 两等差数列{an}{bn}的...的网友还看了以下：