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Multiple choice maths geometric sequences introduction to geometric progression understanding geometric progressions introduction to geometric progressions

How many terms are there in the G.P $3,6,12,24,.........,384$?

  1. $8$
  2. $9$
  3. $10$
  4. $11$
  5. $7$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Here $a=3$ and $r=\cfrac{6}{3}=2$. Let the number of terms be $n$$.
Then, ${t}_{n}=384$ $\Rightarrow$ $a{r}^{n-1}=384$
$\Rightarrow$ $3\times {2}^{n-1}=384$
$\Rightarrow$ ${2}^{n-1}=128={2}^{7}$
$\Rightarrow$ $n-1=7$
$\Rightarrow$ $n=8$
$\therefore$ Number of terms $=8$.

Multiple choice maths geometric sequences introduction to geometric progression understanding geometric progressions introduction to geometric progressions

For a set of positive numbers, consider the following statements:
1. If each number is reduced by $2$, then the geometric mean of the set may not always exists.
2. If each number is increased by $2$, then the geometric mean of the set is increased by $2$.
Which of the above statements is/are correct?

  1. $1$ only
  2. $2$ only
  3. Both $1$ and $2$
  4. Neither $1$ nor $2$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

1. Consider the two numbers $1$ and $4$, geometric mean of $1$ and $4$ is $\sqrt {1 \times 4} = 2$.
When each number is reduced by $2$, the numbers become $-1$ and $2$ whose geometric mean does not exist.
2. Now consider two numbers $2$ and $7$. Their geometric mean is $\sqrt {14}$. The new numbers are $4$ and $9$ whose geometric mean is $\sqrt {4\times 9} = 6$ which is not equal to $2\sqrt {14}$.
Thus only statement $1$ is true.

Multiple choice maths geometric sequences introduction to geometric progression understanding geometric progressions introduction to geometric progressions

If $a, b, c$ are in G.P., then $\dfrac {a - b}{b - c}$ is equal to

  1. $\dfrac {a}{b}$
  2. $\dfrac {b}{a}$
  3. $\dfrac {a}{c}$
  4. $\dfrac {c}{b}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

We are given $a,b,c$ are in G.P.


Hence, ${b}^{2}=a\times c$


$\dfrac { a-b }{ b-c }=\dfrac { a-b }{ b-\dfrac { { b }^{ 2 } }{ a }  } $

$=\dfrac { a\left( a-b \right)  }{ b\left( a-b \right)  } $

$=\dfrac { a }{ b } $

Hence, option A is correct.

Multiple choice maths geometric sequences introduction to geometric progression understanding geometric progressions introduction to geometric progressions

The sum of infinity of $\frac{1}{7} + \frac{2}{7^2} + \frac{1}{7^3} + \frac{2}{7^4} + ......$ is:

  1. $\frac{1}{5}$
  2. $\frac{1}{24}$
  3. $\frac{5}{48}$
  4. $\frac{3}{16}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The series can be split into two geometric series: S1 = 1/7 + 1/7^3 + 1/7^5... and S2 = 2/7^2 + 2/7^4 + 2/7^6... For S1, a=1/7, r=1/49, sum = (1/7)/(1-1/49) = 7/48. For S2, a=2/49, r=1/49, sum = (2/49)/(1-1/49) = 2/48. Total sum = 9/48 = 3/16.

Multiple choice maths geometric sequences introduction to geometric progression understanding geometric progressions introduction to geometric progressions

The limit of the sum of an infinite number of terms in a geometric progression is $a/(1 - r)$ where a denotes the first term and $-1 <r<1$ denotes the common ratio. The limit of the sum of their squares is:

  1. $\dfrac{a^2}{(1 - r)^2}$
  2. $\dfrac{a^2}{1 + r^2}$
  3. $\dfrac{a^2}{1 - r^2}$
  4. $\dfrac{4a^2}{1 + r^2}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

If the original series is a, ar, ar^2, ..., the sum is a/(1-r). The series of squares is a^2, a^2r^2, a^2r^4, ..., which is a geometric series with first term a^2 and common ratio r^2. The sum is a^2/(1-r^2).

Multiple choice maths geometric sequences introduction to geometric progression understanding geometric progressions introduction to geometric progressions

If $S=1+\dfrac{1}{2}+\dfrac{1}{4}+\dfrac{1}{8}+\dfrac{1}{16}+\dfrac{1}{32}+....\infty$.
then, the sum of the given series is $2$.

  1. True

  2. False
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

We have,

$S=1+\dfrac{1}{2}+\dfrac{1}{4}+\dfrac{1}{8}+\dfrac{1}{16}+\dfrac{1}{32}+....\infty$

Then,
$a=1$, $r=\dfrac{1}{2}$

We know that
$S=\dfrac{a}{1-r}$

$S=\dfrac{1}{1-\dfrac{1}{2}}$

$S=\dfrac{1}{\dfrac{1}{2}}$

$S=2$

Hence, this is the answer.

Multiple choice maths geometric sequences introduction to geometric progression understanding geometric progressions introduction to geometric progressions

Given a sequence of $4$ members, first three of which are in G.P. and the last three are in A.P. with common difference six. If first and last terms of this sequence are equal, then the last term is:

  1. $8$
  2. $16$
  3. $2$
  4. $4$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
Let for terms be $a,ar,ar^{2},a$

$\because ar,ar^{2}$ and a are in A.P

$\therefore ar^{2}-ar=6$

$\Rightarrow ar(r-1)=6$

And $a-ar=2\times 6$

$\Rightarrow a(r-1)=-12$

$\Rightarrow \dfrac{ar(r-1)}{a(r-1)}=\dfrac{-6}{12}$

$\Rightarrow r=-\dfrac{1}{2}(\because r\neq 1)$

$\therefore a(1-r)=12$

$\Rightarrow a\left(1+\dfrac{1}{2}\right)=12$

$\Rightarrow \dfrac{39}{2}=12$

$\Rightarrow a=8$

$\therefore $ Last term = $8$