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Determine whether the given sequence is arithmetic, geometric, or neither. If the sequence is arithmetic, find the common difference and the sum of the first n terms. If the sequence is geometric, find the common ratio and the sum of the first n terms.

25,10,4,85,...

Short Answer

Expert verified

25,10,4,85,... is a Geometric Sequence.

Also, Common Ratio, r=25and Sum of first n terms,Sn=12531-25n .

Step by step solution

01

Given information:

Consider the given sequence,25,10,4,85,...

02

Formulas:

ForArithmeticSequence:Commondifference:d=an-an-1ForGeometricSequence:Commonratio:r=anan-1Sn=a11-rn1-r

03

To check whether the given sequence is arithmetic sequence:

To check for arithmetic: Difference of any 2 successive terms is constant, that is common differenced.

The given sequence 25,10,4,85,...

d=10-25=-15d=4-10=-6

∴The given sequence is not an arithmetic sequence.

04

To check whether the given sequence is geometric sequence:

To check for geometric: Ratio of successive terms is constant, that is common ratior.

The given sequence is 25,10,4,85,...

r=1025=25r=410=25r=854=85×14=25

∴The given sequence is a geometric sequence.

AndCommonRatio,r=25.

05

to find the sum of first n terms Sn:

The given sequence is 25,10,4,85,...

a1=25,r=25

role="math" localid="1647508244083" Sn=251-25n1-25=251-25n1×5-25=251-25n×53=12531-25n

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