已知等差数列an=4n-2,等比数列bn=2/4(n-1)(四的n减1次方分之二),设cn=an/bn,求cn前n项和T
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已知等差数列an=4n-2,等比数列bn=2/4(n-1)(四的n减1次方分之二),设cn=an/bn,求cn前n项和Tn.
![已知等差数列an=4n-2,等比数列bn=2/4(n-1)(四的n减1次方分之二),设cn=an/bn,求cn前n项和T](/uploads/image/z/15745009-49-9.jpg?t=%E5%B7%B2%E7%9F%A5%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97an%3D4n-2%2C%E7%AD%89%E6%AF%94%E6%95%B0%E5%88%97bn%3D2%2F4%28n-1%29%EF%BC%88%E5%9B%9B%E7%9A%84n%E5%87%8F1%E6%AC%A1%E6%96%B9%E5%88%86%E4%B9%8B%E4%BA%8C%EF%BC%89%2C%E8%AE%BEcn%3Dan%2Fbn%2C%E6%B1%82cn%E5%89%8Dn%E9%A1%B9%E5%92%8CT)
cn=an/bn=[(4n-2)/2]*4^(n-1)=(2n-1)*4^(n-1)
Tn=1+3*4+5*4^2+.+(2n-1)*4^(n-1)
4Tn=4+3*4^2+5*4^3+...+(2n-1)*4^n
Tn-4Tn=1+2*4+2*4^2+...+2*4^(n-1)-(2n-1)*4^n
-3Tn=1+2*4*[4^(n-1)-1]/(4-1)-(2n-1)*4^n
=1+(8/3)*[4^(n-1)-1]-(2n-1)*4^n
=1+(2/3)*4^n-8/3-(2n-1)*4^n
Tn=[(2n-1)/3-2/9]*4^n+5/9
=(1/9)*[(6n-5)*4^n+5]
Tn=1+3*4+5*4^2+.+(2n-1)*4^(n-1)
4Tn=4+3*4^2+5*4^3+...+(2n-1)*4^n
Tn-4Tn=1+2*4+2*4^2+...+2*4^(n-1)-(2n-1)*4^n
-3Tn=1+2*4*[4^(n-1)-1]/(4-1)-(2n-1)*4^n
=1+(8/3)*[4^(n-1)-1]-(2n-1)*4^n
=1+(2/3)*4^n-8/3-(2n-1)*4^n
Tn=[(2n-1)/3-2/9]*4^n+5/9
=(1/9)*[(6n-5)*4^n+5]
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