Question Details

Let a1, a2,……..a3n be an arithmetic progression with a1 = 3 and a2 = 7. If a1 + a2 + ….+a3n = 1830, then what is the smallest positive integer m such that m (a1 + a2 + …. + an) > 1830?

Options

A

8

B

9

C

10

D

11

Show Answer

Correct Answer :

Option B

9

Solution :

The correct option is B.

Given the first term of the arithmetic progression (AP) is a1=3 and the second term is a2=7.
The common difference d of the AP is:
d=a2a1=73=4

The sum of the first 3n terms is given as 1830:
S3n=3n2[2a1+(3n1)d]=< 1830
Substitute a1=3 and d=4 into the equation:
3n2[2(3)+(3n1)4]=1830
3n2[6+12n4]=1830
3n2[12n+2]=1830
3n(6n+1)=1830
n(6n+1)=610
6n2+n610=0

Solving the quadratic equation:
(6n+61)(n10)=0
Since n must be a positive integer, we have n=10.

Now, let's find the sum of the first n terms, i.e., S10:
Sn=a1+a2++an
S10=102[2(3)+(101)4]
S10=5[6+36]=5(42)=210

We need to find the smallest positive integer m such that:
m(Sn)>1830
m(210)>1830
m>1830210
m>8.71
Thus, the smallest positive integer m satisfying this inequality is 9.

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