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Question: Answered & Verified by Expert
Let the positive numbers a1, a2, a3, a4 and a5 be in a G.P. Let their mean and variance be 3110 and mn respectively, where m and n are co-prime. If the mean of their reciprocals is 3110 and a3+a4+a5=14, then m+n is equal to ____________.
MathematicsStatisticsJEE MainJEE Main 2023 (12 Apr Shift 1)
Solution:
2291 Upvotes Verified Answer
The correct answer is: 211

5 positive numbers a1, a2,..., a5 are in geometric progression. So, let the numbers be ar2, ar, a, ar, ar2

According to question,

ar2+ar+a+ar+ar25=3110   ...1

And,

r2a+ra+1a+1ar+1ar25=3140

1ar2+r+1+1r+1r25=3140   ...2

By 1 & 2, we get

a2=4 a=2

So,

r2+r+1+1r+1r2=314

r+1r2+r+1r=274+2

t2+t=354

where, t=r+1r

4t2+4t-35=0

t=52r=2

So, numbers are 12, 1, 2, 4, 8

Variance is

=14+1+4+16+645-12+1+2+4+852

=18625=mn

m+n=211

Hence this is the required option.

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