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JEE Advanced · Mathematics · 5. Sequences & Series

Let l1,l2...l100 be consecutive terms of an arithmetic progression with common difference d1, and let w1,w2, ,w100 be consecutive terms of another arithmetic progression with common difference d2, where d1d2=10. For each i=1,2,3..........100, let Ri be a rectangle with length li, width wi and area Ai.If A51-A50=1000, then the value of A100-A90 is _______.

  1. A 18900
  2. B 19000
  3. C 12450
  4. D 19124
Verified Solution

Answer & Solution

Correct Answer

(A) 18900

Step-by-step Solution

Detailed explanation

Given \(l_1, l_2 \ldots l_{100}\) are consecutive terms of an \(A . P\)
Now let \(T_1=a\) and common difference \(=d_1\)
And similarly for \(A\). \(P w_1, w_2, \ldots w_{100}, T_1=b\) and common difference \(=d_2\)
Now given, \(A_{51}-A_{50}=l_{51} w_{51}-l_{50} w_{50}\)
\(\Rightarrow\left(a+50 d_1\right)\left(b+50 d_2\right)-\left(a+49 d_1\right)\) \(\left(b+49 d_2\right)=1000 \)
\( \Rightarrow 50 b d_1+50 a d_2+2500 d_1 d_2-49 a d_2~-\) \(49 b d_1-2401 d_1 d_2=1000 \)
\( \Rightarrow b d_1+a d_2+99 d_1 d_2=1000\)
So, \(b d_1+a d_2=10\left\{\right.\) as given \(\left.d_1 d_2=10\right\}\)
Now finding \(A_{100}-A_{90}=l_{100} w_{100}-l_{90} w_{90}\) we get,
\(=\left(a+99 d_1\right)\left(b+99 d_2\right)-\left(a+89 d_1\right)\) \(\left(b+89 d_2\right) \)
\( =99 b d_1+99 a d_2+99^2 d_1 d_2-89 b d_1\) \(-~89 a d_2-89^2 d_1 d_2 \)
\( =10\left(b d_1+a d_2\right)+1880 d_1 d_2 \)
\(=10(10)+18800 \text { \{again using } d_1 d_2=10\) & \(b d_1+a d_2=10\} \)
\( =18900\)
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