已知数列an的前n项和sn=n^2+pn,数列bn的前n项和Tn=2bn-1,求数列{an*bn}的前n项和Mn

2个回答

  • a1=S1=1+p

    n≥2时,

    an=Sn-Sn-1

    =n²+pn-(n-1)²-p(n-1)

    =2n-1+p

    b1=T1=2b1-1

    b1=1

    n≥2时,

    bn=Tn-Tn-1

    =2bn-1-2bn-1 + 1

    =2bn-2bn-1

    推出bn=2bn-1

    所以bn=b1×2^(n-1)=2^(n-1)

    设cn=an×bn=(2n-1+p)×2^(n-1),Un为{cn}前n项和.

    则2Un-Un=Un

    = (1+p)×2+(3+p)×2^2+(5+p)×2^3+(7+p)×2^4+.+(2n-5+p)×2^(n-2)+(2n-3+p)×2^(n-1)+(2n-1+p)×2^n

    -[(1+p)+(3+p)×2+(5+p)×2^2+(7+p)×2^3+(9+p)×2^4+.+(2n-3+p)×2^(n-2)+(2n-1+p)×2^(n-1)]

    =-(1+p)-2×2-2×2^2-2×2^3-2×2^4-.-2×2^(n-2)-2×2^(n-1)+(2n-1+p)×2^n

    =-(1+p)-[2^2+2^3+2^4+.+2^(n-1)]+(2n-1+p)×2^n

    =-1-p-[4-2^(n+1)]/(1-2)+(2n-1+p)×2^n

    =4-2^(n+1)-1-p+(2n-1+p)×2^n

    =-2×2^n+(2n-1+p)×2^n+3-p

    =(2n-3+p)×2^n+3-p