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Q.

Let α and β be the roots of x2x1=0 with α>β . For all positive integers n, define an=αnβnαβ,          n1b1=1 and bn=an1+an+1,n2 Then which of the following options is/are correct?

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a

n=1bn10n=889

b

bn=αn+βn

c

a1+a2+a3+......an=an+21

d

n=1an10n=1089

answer is A, B, D.

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Detailed Solution

Given α,β  are the roots of the equation x2x1=0hence, α=1+52, β=152

Given an=αnβnαβ,n1Given, b1=1  and bn=an1+an+1=αn1βn1αβ+αn+1βn+1αβ=αn1α2+1βn1β2+1αβ=αn1α+2βn1β+2αβ=αn15+52βn15525=αn1αβn1β=αn+βnand n=1bn10n=n=1αn+βn10n=n=1α10n+n=1β10n

=α101-α10+β101-β10=α10-α+β10-β=10(α+β)-2αβ100-10(α+β)+αβ=10(1)-2-1100-10+(-1)=1289n=1an10n=n=1αn-βn(α-β)10n=15n=1α10n-n=1β10n=15α10-α-β10-β=1510(α-β)100-10(α+β)+αβ=1510×589=1089  Also a1+a2+a3+.+an=1α-β(α-β)+α2-β2+α3-β+.+αn-βn=1α-βα+α2+..+αn-β+β2+βn=1α-β1+α+α2+αn-1+β+β2.βn=1α-βαn+1-1α-1-βn+1-1β-1=αn+2-βn+2α-β-1=an+2-1

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