Sequence And Series (II) – Geometric Progression

CONTENT

• Nth term of a G.P
• Geometric Mean
• Sum of n terms of a G.P
• Sum to infinity of a G.P

Nth term of a G.P

A Geometric Progression is a sequence generated by multiplying or dividing a preceding term by a constant number to get a term. This constant number is called common ratio designated by letter r.

Geometric Mean

Suppose x, y, z are consecutive terms of a geometric progression, then the common ratio r can be written as:

                        r = y/x = z/y

                        y/x = z/y

                              y² = xz

                               y =   √xz                         y is the geometric mean of x and z.

Evaluation

1. The second and fourth terms of a G.P are 21 and 189. Find the sum of the first seven terms.
2. Find the sum to infinity of 1+¹/₃ + ¹/₉ + ¹/₂₇ …………

GENERAL  EVALUATION

1.   Find the (a)sum of the first 8 terms  (b)sum to infinity of the series:  -5 , 5/2,  -5/4 , 5/8…….

2.   The sum of the first two terms of a G.P is 2½ and the sum of the first four terms is 3¹¹/18.

      Find    the G.P if r > 0.

3.   Solve the following exponential equations  (a) 2^2x -6(2^x) + 8 = 0        (b) 2^2x+1 -5(2^x) + 2 = 0

READING ASSIGNMENT: Further Mathematics Project Book 1(New third edition).Chapter 33-36 & 37-45

WEEKEND ASSIGNMENT

1.          The sum to infinity of a G.P is 60. If the first term of the series 12, find its second term of the series 12, find its second term. A.   9.6        B.    6.9        C.  12.6       D.  8.6

2.          A G.P has 6 terms. If the 3^rd and 4^th terms are 28 and -56 respectively, find the sum of the G. P.

            A.   471         B.   -471            C.   – 147        D. -741

3.          Find the sum of the G.P 2 + 6 + 18 + 54 + …………1458.  A. 8216   B. 6218   C.1682    D. 2186

4.          The 8^th term of a G.P is -7/32. Find its common ratio if its first term is 28.

            A. ½     B.   -½      C.  –²/₃       D.   ³/₂

5.          Given the geometric progression 5, 10, 20, 40, 80 …………….. find its nth term.

            A.   2(5ⁿ⁺²)        B.    5(2ⁿ⁺¹)          C.   5(2^n-1)           D.  2(5^n-1)

          THEORY

1.          The fifth term of a G.P is greater than the fourth term by 13½, and the fourth term is greater than the third by 9. Find (i)  the common ratio   (ii)  the first term

2.          The sum of the first two terms of an exponential sequence is 135 and the sum of the third and the fourth terms is 60. Given that the common ratio is positive, calculate

(i)         the common        (ii)       the limit of the sum of the first n terms as n becomes large

(iii)        the least number of terms for which the sum exceeds 240