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SOLVING GEOMETRIC SEQUENCES

Updated on April 28, 2011

SOLVING GEOMETRIC SEQUENCES

Arithmetic sequences are formed by addition, whereas geometric sequences are formed by multiplication. Geometric sequences are also called geometric progressions .

A geometric sequence is one in which each term is multiplied by the same number to get the next term. This number is known as the common ratio r,

where r * An = An + 1 for n = 1, 2, 3, ..........

r maybe positive or negative.

Problem One :

Verify whether each of the following sequences is actually a geometric sequence.

(a) 5,000 20,000 80,000 32,0000

Here A = 5000 , r = 20,000/5,000 = 4

(b) 10, 000 5,000 2,500 1,250

A = 10,000 r = 5,000/10,000 = ½

(c ) 1,000 2,200 4,840 10,648

A = 1000 r = 2,200/1,000 = 2.2

All of the above are examples of geometric sequences.

The formula for finding the nth term of a certain geometric progression is given as :

An = A r ^( n-1)

Where A = first term

r = common ratio

r = A2/A1 = A3/A2 = An + 1/An

n = number of terms

An = the nth term

Problem Number Two :

Find the eighth term of the geometric sequence which begins with ¾ and 3/5.

Solution :

The ratio is : r = 3/5 ÷ ¾ = 4/5

A = ¾ n = 8 , n - 1 = 8 - 1 = 7

Substituting to the formula above :

A 8 = ¾ * (4/5) ^7

= ¾ * 16,384 / 78,125

= [(4,096 ) *3 ] / 78, 125

= 12,288/78,125

The formula for finding the sum (Sn) of the first n terms of a geometric sequence with first term A and common ratio r, where r should not be equal to 1 is given as :

Sn = [ A (r ^n - 1 ) ]/ r - 1

Problem Number Three :

Find the sum of the first ten terms of the geometric series starting with -5 and 15.

Solution :

r = 15/-5 = -3

A = -5

n = 10

Sn = [ -5 ( -3 ^10 - 1)] / -3-1

= [ -5 ( 59,049 - 1)] /-4 =-5(59,048)/-4 = -295,240/-4 = 73,810

Alternative formula for Sn :

Sn = ( A - r An ) / 1 - r

Problem Number Four :

The first term of a geometric sequence is 5 and the fourth term is -320.

Find the eighth term and the sum of the first eight terms.

Solution :

We are given with A = 5, if we first use n = 4 in the formula

An = A r^( n - 1) we obtain,

-320 = 5 r ^3

r ^3 = - 320/5 = -64

r = - 4

We next use n = 8 in the formula for An and Sn

A 8 = 5 ( -4 )^ 7 = 5 (-16, 384) = -81, 920

S 8 = (A - r An )/ 1 - r

= [ 5 - (-4) (-81,920)] /1 - (-4)

= [5 - 327, 680 ] /5

= -327, 675 / 5

= -65, 535

Problem Number Five:

Find r and A if S5 = 1, 563 and A5 = 1, 875

Solution :

First, we use the formula for An :

1,875 = A r^ 4 let this be equation (1 ).

Then we use the formula for Sn,

1, 563 = (A - 1,875 r) / 1 - r let this be equation (2)

Solving the second equation for A we obtain,

( 1 - r ) (1,563) = A - 1, 875 r

1,563 - 1,563r = A - 1, 875 r

312 r + 1,563 = A or A = 312 r + 1, 563, let this be equation (3)

Substituting this value in the first equation, we now have

1,875 = (312 r + 1,563) r^ 4

1, 875 = 312 r^ 5 + 1,563 r^ 4 0r 312 r^ 5 + 1, 563 r^ 4 - 1, 875 = 0

By using the theorem on rational zeros of polynomial function, we find one of the

Solution to be r = -5.

Substituting r = -5 in the equation (3)

A = 1, 563 + (312) ( -5)

A = - 1,560 + 1, 563 = 3

SOURCE : COLLEGE ALGEBRA

Paul K. Rees

Fred W. Sparks

Charles Sparks Rees

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    • profile image

      Qt 8 years ago

      that's too hard Are you a relative of albert einstein ?

    • profile image

      thats my name 7 years ago

      you can also multiply the first number and the third then square it to get the second..

    • profile image

      patrick teng 7 years ago

      wow i know you are the relative of albert einstein!

    • profile image

      April S 6 years ago

      Geometric Sequences are pretty interesting. I like figuring out the common ratio and seeing the pattern emerge!

    • profile image

      jd 5 years ago

      HELP! I have this problem; find the next two terms in the sequence 1, 8 , 18, 31, 47..,..

      I need help with finding the raito. 8 divided by 1 is ok, but if you go 18 divided by 8 then what? the ratio isn't the same for all of them.

      Your help would be greatly appreciated!!

    • profile image

      vamsee 5 years ago

      nice

    • Rrudz profile image

      Rrudz 5 years ago from london

      Can any 1 give the ans for this ques ...............S6=162 and S8=4374 find series (GP)

    • profile image

      iglot 5 years ago

      pahelp naman po,,

      1. find the three geometric sequence whose sum is 26 and the product is 216

      2.a geometric sequence consist of 4 terms in which the sum of the first two terms is 8, and the sum of the last two terms is 72

    • profile image

      Atiah Daniel 5 years ago

      The third term of an exponetial squence is 10

    • profile image

      joan mae 5 years ago

      pa help nmn poh panu mag solve nang geometric sequence ..'hnd q po kc makuha panu i solve eh

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