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JEE Advanced · Mathematics · 5. Sequences & Series

Let bi>1 for i=1, 2,.,101. Suppose logeb1,logeb2,..,logeb101 are in Arithmetic Progression (A.P.) with the common difference loge2.  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

  1. A s>t and a101>b101
  2. B s>t and a101<b101
  3. C s<t and a101>b101
    s<t and a101>b101
  4. D s<t and a101<b101
    s<t and a101<b101
Verified Solution

Answer & Solution

Correct Answer

(B) s>t and a101<b101

Step-by-step Solution

Detailed explanation

If \(\log _e b_1, \log _e b_2 \ldots . \log _e b_{101} \rightarrow A P ; \quad\) difference \(( d )=\log _e 2\)
b 1 , b 2 , b 3 ....... b 101 GP;r=2
b 1 ,2 b 1 , 2 2 b 1 ........., 2 100 b 1 GP
a 1 , a 2 , a 3 .......... a 101 AP Let Common Difference = D
Given, a1=b1 and a51=b51
\( \Rightarrow a_1+50 D=2^{50} b_1 \)
\( \therefore a_1+50 D=2^{50} a_1\left(A s b_1=a_1\right) \Rightarrow\) \(D=\frac{2^{50} a_1-a_1}{50} \)
\( t=b_1+b_2+\ldots \ldots b_{51}=b_1+2 b_1\) \(+~2^2 b_1+\ldots \ldots 2^{50} b_1 \Rightarrow t=b_1\left(2^{51}-1\right) ; \)
\( s=a_1+a_2+\ldots \ldots a_{51} \Rightarrow s=\frac{51}{2}(2a_1\) \(+~50 D) \)
\( t=a_1 \cdot 2^{51}-a_1 \Rightarrow t1 \text { \& } a_1=b_1\) so \(a_1>1) \)
\( s=\frac{51}{2}\left(a_1+a_1+50 D\right) \)
\( s=\frac{51}{2}\left(a_1+2^{50} a_1\right) \)
\( s=\frac{51 a_1}{2}+\frac{51}{2} \cdot 2^{50} a_1 \)
\( \Rightarrow s>a_1 \cdot 2^{51} \ldots \ldots \text { (ii) }\)
Clearly s>t (from equation (i) and (ii))
Also a101=a1+100D;  b101=b1. 2100
\(\therefore a_{101}=a_1+100\left(\frac{2^{50} a_1-a_1}{50}\right) ; b_{101}\) \(=2^{100} a_1 \ldots \ldots \text { (ii) }\)
\(a_{101}=a_1+2^{51} a_1-2 a_1 \Rightarrow a_{101}=\) \(2^{51} a_1-a_1 \Rightarrow a_{101}<2^{51} a_1 \ldots \ldots .( iv )\)
Clearly b101>a101 (from equation (iii) and (iv))
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