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Sequences and Series question

2023 · 29 Jan · Shift 1 · Q41
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Sequences and Series question

2023 · 29 Jan · Shift 1 · Q41

JEE MainMathematicsSequences and SeriesNumerical+4 / −1
Let a1,a2,a3,...a_1,a_2,a_3,...a1​,a2​,a3​,... be a GPGPGP of increasing positive numbers. If the product of fourth and sixth terms is 9 and the sum of fifth and seventh terms is 24, then a1a9+a2a4a9+a5+a7a_1a_9+a_2a_4a_9+a_5+a_7a1​a9​+a2​a4​a9​+a5​+a7​ is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
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Correct answer: 60

Let the GP be an=arn−1a_n=ar^{n-1}an​=arn−1 with a>0a>0a>0 and r>1r>1r>1 since the terms are increasing positive numbers.

We are given:

  1. Product of 4th and 6th terms is 999: a4a6=(ar3)(ar5)=a2r8=9a_4a_6=(ar^3)(ar^5)=a^2r^8=9a4​a6​=(ar3)(ar5)=a2r8=9 But a5=ar4a_5=ar^4a5​=ar4 so a52=a2r8=9 ⇒ a5=3a_5^2=a^2r^8=9 \,\Rightarrow\, a_5=3a52​=a2r8=9⇒a5​=3 (since all terms are positive).

  2. Sum of 5th and 7th terms is 242424: a5+a7=ar4+ar6=ar4(1+r2)=24a_5+a_7=ar^4+ar^6=ar^4(1+r^2)=24a5​+a7​=ar4+ar6=ar4(1+r2)=24 Using a5=ar4=3a_5=ar^4=3a5​=ar4=3, 3(1+r2)=243(1+r^2)=243(1+r2)=24 1+r2=81+r^2=81+r2=8 r2=7r^2=7r2=7 Since r>1r>1r>1, r=7r=\sqrt{7}r=7​

Now compute the required expression: a1a9+a2a4a9+a5+a7a_1a_9+a_2a_4a_9+a_5+a_7a1​a9​+a2​a4​a9​+a5​+a7​

Step 1: Find a1a9a_1a_9a1​a9​

a1a9=a⋅ar8=a2r8=9a_1a_9=a\cdot ar^8=a^2r^8=9a1​a9​=a⋅ar8=a2r8=9

Step 2: Find a2a4a9a_2a_4a_9a2​a4​a9​

a2a4a9=(ar)(ar3)(ar8)=a3r12a_2a_4a_9=(ar)(ar^3)(ar^8)=a^3r^{12}a2​a4​a9​=(ar)(ar3)(ar8)=a3r12 Rewrite as a3r12=(ar4)3=a53=33=27a^3r^{12}=(ar^4)^3=a_5^3=3^3=27a3r12=(ar4)3=a53​=33=27

Step 3: Find a5+a7a_5+a_7a5​+a7​

This is already given: a5+a7=24a_5+a_7=24a5​+a7​=24

Step 4: Add all parts

a1a9+a2a4a9+a5+a7=9+27+24=60a_1a_9+a_2a_4a_9+a_5+a_7=9+27+24=60a1​a9​+a2​a4​a9​+a5​+a7​=9+27+24=60

Therefore, the required integer is 60\boxed{60}60​

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