arthham

JEE Advanced 2016 · Sequences and Series

Let bi>1b_i > 1 for i=1,2,…,101i = 1, 2, \ldots, 101. Suppose log⁡eb1,log⁡eb2,…,log⁡eb101\log_e b_1, \log_e b_2, \ldots, \log_e b_{101} are in Arithmetic Progression (A.P.) with the common difference log⁡e2\log_e 2. Suppose a1,a2,…,a101a_1, a_2, \ldots, a_{101} are in A.P. such that a1=b1a_1 = b_1 and a51=b51a_{51} = b_{51}. If t=b1+b2+…+b51t = b_1 + b_2 + \ldots + b_{51} and s=a1+a2+…+a51s = a_1 + a_2 + \ldots + a_{51}, then

Answer: B. s>ts > t and a101<b101a_{101} < b_{101}

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Written solution

Let bᵢ > 1 for i = 1, 2, …, 101. Suppose logₑ b₁, logₑ b₂, …, logₑ b₁₀₁ are in Arithmetic Progression (A.P.) with the common difference logₑ 2. Suppose a₁, a₂, …, a₁₀₁ are in A.P. such that a₁ = b₁ and a₅₁ = b₅₁. If t = b₁ + b₂ + … + b₅₁ and s = a₁ + a₂ + … + a₅₁, then

bi>1,i=1,2,…,101b_i > 1,\quad i = 1, 2, \ldots, 101

log⁡eb1,log⁡eb2,…,log⁡eb101 in A.P.\log_e b_1, \log_e b_2, \ldots, \log_e b_{101} \ \text{in A.P.}

common difference d=log⁡e2\text{common difference } d = \log_e 2

a1=b1,a51=b51a_1 = b_1,\qquad a_{51} = b_{51}

t=b1+b2+…+b51t = b_1 + b_2 + \ldots + b_{51}

s=a1+a2+…+a51s = a_1 + a_2 + \ldots + a_{51}

log⁡ebi=log⁡eb1+(i−1)log⁡e2\log_e b_i = \log_e b_1 + (i-1)\log_e 2

⇒ bi=b1⋅2 i−1\Rightarrow\ b_i = b_1 \cdot 2^{\,i-1}

so bi is a G.P. with common ratio 2\text{so } b_i \text{ is a G.P. with common ratio } 2

b51=b1⋅250b_{51} = b_1 \cdot 2^{50}

d=a51−a150=b1(250−1)50d = \dfrac{a_{51} - a_1}{50} = \dfrac{b_1(2^{50} - 1)}{50}

ai=a1+(i−1)da_i = a_1 + (i-1)d

ai=b1+(i−1) b1(250−1)50a_i = b_1 + (i-1)\,\dfrac{b_1(2^{50} - 1)}{50}

the chord ai lies above the convex bi\text{the chord } a_i \text{ lies above the convex } b_i

ai≥bi(1<i<51)a_i \ge b_i \quad (1 < i < 51)

s−t=∑i=151(ai−bi)>0s - t = \sum_{i=1}^{51}(a_i - b_i) > 0

∴ s>t\therefore\ s > t

t=b1+b12+…+b1250t = b_1 + b_1 2 + \ldots + b_1 2^{50}

t=b1 251−12−1=b1(251−1)t = b_1\,\dfrac{2^{51}-1}{2-1} = b_1(2^{51}-1)

s=512(a1+a51)=512 b1(1+250)s = \dfrac{51}{2}(a_1 + a_{51}) = \dfrac{51}{2}\,b_1(1 + 2^{50})

s−t=b1 ⁣(532+472⋅250)>0s - t = b_1\!\left(\dfrac{53}{2} + \dfrac{47}{2}\cdot 2^{50}\right) > 0

a101=a1+100 da_{101} = a_1 + 100\,d

=b1+100⋅b1(250−1)50= b_1 + 100\cdot\dfrac{b_1(2^{50}-1)}{50}

=b1(251−1)= b_1(2^{51}-1)

b101=b1⋅2100b_{101} = b_1 \cdot 2^{100}

a101=b1(251−1)<b1⋅2100=b101a_{101} = b_1(2^{51}-1) < b_1\cdot 2^{100} = b_{101}

∴ a101<b101\therefore\ a_{101} < b_{101}

s>tanda101<b101s > t \quad \text{and} \quad a_{101} < b_{101}

Answer: (2)\boxed{\text{Answer: (2)}}

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