2 Find the least positive integer n such that 1+6+62 + ... +6" > 5000
the least number of terms for which the sum must be g
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Step-by-step explanation:
We know that the sum of a G.P. is a(r-1)^n/r–1 for r>1.
Putting values along with condition:
1(6–1)^n/6–1 > 5000
5^n/5> 5000
5^(n-1) > 5000
Diving both sides by 5,
5^(n-2) > 1000
5^(n-2) > 125*8
5^(n-2) > 5^3*8
5^(n-5) > 8
Taking log both sides,
Log(5^(n-5)) > Log(2^3)
(n-5)Log5> 3Log2
n-5 > 3Log2/Log5
Putting values of Log2 and Log5,
n-5> 3*(0.301)/0.698
n-5> 1.3
n> 6.3
But n is an integer, so,
n = 7
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