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Q.

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 3140 and a3+a4+a5=14, then m + n is equal to_________.

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answer is 211.

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Detailed Solution

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a(1r2+1r+1+r+r2)=312 1a(1r2+1r+1+r+r2)=318 a2=4a=2 again 1r2+1r+1+r+r2=314 (r+1r)2+(r+1r)=314+1=354 Put r+1r=t then, 4t2+4t35=0 t=5/2         r=2
Numbers are 12,1,2,4,8
σ2=14+1+4+16+645(3110)2=34120961100 σ2=18625=mn m+n=211

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