已知等差数列AP :{an}=2n-1
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已知等差数列AP :{an}=2n-1
已知等比数列GP:{bn}=2^(n+1)
求T=a1b1+a2b2+a3b3+.+anbn=_________
已知等差数列AN的通项公式为2n-1,等比数列BN的通项公式为2的n+1次方,求a1b1加a2b2加a3b3加加加.加到anbn的值为多少?
已知等比数列GP:{bn}=2^(n+1)
求T=a1b1+a2b2+a3b3+.+anbn=_________
已知等差数列AN的通项公式为2n-1,等比数列BN的通项公式为2的n+1次方,求a1b1加a2b2加a3b3加加加.加到anbn的值为多少?
错位相消法:
Tn=1×2²+3×2³+ … +(2n-3)×2^n+(2n-1)×2^(n+1) ①式;
等式两边同乘2,得2Tn= 1×2³+3×2^4+…+(2n-5)×2^n+(2n-3)×2^(n+1)+(2n-1)×2^(n+2) ②式;
①-②,得-Tn=1×2²+2×2³+2×2^4+……+2×2^n+2×2^(n+1)-(2n-1)×2^(n+2)
=4+2^4+2^5+………+2^(n+2)-(2n-1)×2^(n+2)
=4+2^(n+2)-2^4-(2n-1)×2^(n+2)
则Tn=(n-1)×2^(n+3)+12
Tn=1×2²+3×2³+ … +(2n-3)×2^n+(2n-1)×2^(n+1) ①式;
等式两边同乘2,得2Tn= 1×2³+3×2^4+…+(2n-5)×2^n+(2n-3)×2^(n+1)+(2n-1)×2^(n+2) ②式;
①-②,得-Tn=1×2²+2×2³+2×2^4+……+2×2^n+2×2^(n+1)-(2n-1)×2^(n+2)
=4+2^4+2^5+………+2^(n+2)-(2n-1)×2^(n+2)
=4+2^(n+2)-2^4-(2n-1)×2^(n+2)
则Tn=(n-1)×2^(n+3)+12
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