Tag: sequence, progression and series

Questions Related to sequence, progression and series

The value of $3 - 1 + \frac{1}{3} - \frac{1}{9} +  \ldots $ is equal to

  1. $\dfrac{{20}}{9}$

  2. $\dfrac{{9}}{20}$

  3. $\dfrac{{9}}{4}$

  4. $\dfrac{{4}}{9}$


Correct Option: C

Let $P = 3^{1/3} . 3^{2/9} . 3^{3/27} ...\infty$, then $P^{1/3}$ is equal to

  1. $3^{2/3}$

  2. $\sqrt {3}$

  3. $3^{1/3}$

  4. $3^{1/4}$


Correct Option: D
Explanation:

Above in an infinite A.G.S. with $a = 1, d = 1$ for A.P., $b = \dfrac {1}{3}, r = \dfrac {1}{3}$ for G.P.
$\therefore S _{\infty} = \dfrac {ab}{1 - r} + \dfrac {dbr}{(1 - r)^{2}} = \dfrac {\dfrac {1}{3}}{1 - \dfrac {1}{3}} + \dfrac {1 . \dfrac {1}{3} . \dfrac {1}{3}}{\left (1 - \dfrac {1}{3}\right )^{2}} = \dfrac {1}{2} + \dfrac {1}{4} = \dfrac {3}{4}$
$\therefore P = 3^{S} = 3^{3/4} \therefore P^{1/3} = 3^{1/4}$.

The first term of a $G.P.$ whose second term is $2$ and sum to infinity is $8$ will be

  1. $6$

  2. $3$

  3. $4$

  4. $1$


Correct Option: C
Explanation:

$ar = 2, \dfrac {a}{1 - r} = \dfrac {1}{8}$. Eliminate $r$ and we get
$a^{2} - 8a + 16 = 0 \therefore (a - 4)^{2} = 0$ or $a = 4$

The value of $9^{1/3}\times 9^{1/9} \times 9^{1/27} \times .....\infty$ is

  1. $9$

  2. $1$

  3. $3$

  4. None of these


Correct Option: C
Explanation:

$(9)^{S _{\infty}} = 9^{1/2} = 3\because S _{\infty} = \dfrac {1/3}{1 - (1/3)} = \dfrac {1}{2}$.

The value of $9^\cfrac{1}{3}.9^\cfrac{1}{9}.9^\cfrac{1}{27}...........$ upto $\infty$, is

  1. $1$

  2. $3$

  3. $9$

  4. $None\ of\ these$


Correct Option: B
Explanation:
${9^{\cfrac{1}{3}}}{.9^{\cfrac{1}{9}}}{.9^{\cfrac{1}{{27}}}} -  -  -  - upto\,\,\infty $
$ = {9^{\left( {\cfrac{1}{3} + \cfrac{1}{9} + \cfrac{1}{{27}} +  -  -  - } \right)}}$
$ = {9^{\left( {\cfrac{{\cfrac{1}{3}}}{{1 - \cfrac{1}{3}}}} \right)}}$
$ = {9^{\left( {\cfrac{{\cfrac{1}{3}}}{{\cfrac{2}{3}}}} \right)}}$
$ = {9^{\cfrac{1}{2}}}$
$ = 3$

If $x=1+a+{ a }^{ 2 }+{ a }^{ 3 }+....$ to $\infty \left( \left| a \right| <1 \right) $ and 
$y=1+b+{ b }^{ 2 }+{ b }^{ 3 }+...$ to $\infty \left( \left| b \right| <1 \right) $ then
$1+ab+{ a }^{ 2 }{ b }^{ 2 }+{ a }^{ 3 }{ b }^{ 3 }+...$ to $\infty =\cfrac { xy }{ x+y-1 } $

  1. True

  2. False


Correct Option: A
Explanation:

${ x }=\dfrac { 1 }{ 1-a } $ ${ y }=\dfrac { 1 }{ 1-b } $ [summing infinite $G.P's$].
$\therefore a=\dfrac { x-1 }{ x } $, $b=\dfrac { y-1 }{ y } $
$\therefore 1+ab+{ a }^{ 2 }{ b }^{ 2 }+...\infty $
$=\dfrac { 1 }{ 1-ab } =\dfrac { 1 }{ 1-\dfrac { (x-1)(y-1) }{ xy }  } =\dfrac { xy }{ x+y-1 }. $

The sum to infinity of the series $1 + \dfrac{2}{3} + \dfrac{6}{{{3^2}}} + \dfrac{{10}}{{{3^3}}} + \dfrac{{14}}{{{3^4}}} + ......,is$

  1. $3$

  2. $4$

  3. $6$

  4. $2$


Correct Option: A
Explanation:
Let $S$=$1 + \cfrac{2}{3} + \cfrac{6}{{{3^2}}} + \cfrac{{10}}{{{3^3}}} + ......$

$\cfrac{S}{3} = \cfrac{1}{3} + \cfrac{2}{{{3^2}}} + \cfrac{6}{{{3^3}}} + ....$

$S - \cfrac{S}{3} = 1 + \cfrac{1}{3} + \cfrac{4}{{{3^2}}} + ....$

$\cfrac{{2S}}{3} = \cfrac{{\cfrac{4}{3} }}{{1 - \cfrac{1}{3}}}$

$ = \dfrac{\cfrac{4}{3}} { \cfrac{2}{3}} = \cfrac{4}{2} = 2$

If $x = 1\, + a + {a^2} + ......\infty $, $y = 1\, + b + {b^2}\,\, + ......\infty $ where $\left| a \right| < 1$ and $\left| b \right| < 1$, then $\left( {1 + ab + {a^2}{b^2} + ........\infty } \right) = ?$

  1. $\frac{xy}{x+y}$

  2. $\frac{x+y}{xy}$

  3. $\frac{xy}{x+y+1}$

  4. $\frac{xy}{x+y-1}$


Correct Option: D

Value of $y = {\left( {0.64} \right)^{{{\log } _{0.25}}\left( {\cfrac{1}{3} + \cfrac{1}{{{3^2}}} + \cfrac{1}{{{3^3}}}....upto   \infty } \right)}}$ is :

  1. $0.9$

  2. $0.8$

  3. $0.6$

  4. $0.25$


Correct Option: B
Explanation:
$y= (0.64)^{log _{0.25} \left(\dfrac{1}{3}+ \dfrac{1}{3^{2}}+ \dfrac{1}{3^{3}}+..... \right)}$
$=(0.64)^{\log _{0.25}^{\left( \dfrac{\dfrac{1}{3}}{1-1/3} \right)}}$
$=(0.64)^{\log _{0.25} } \left( \dfrac{1}{2} \right)$
$= (0.64)^{\log 0.5} _{0.25}$
$(0.64)^{0.5}= (0.64)^{1/2}= \sqrt{0.64}= 0.8$

If $y=x-x^2+x^3-x^4+....\infty$, then value of x will be?

  1. $y+\dfrac{1}{y}$

  2. $\dfrac{y}{1+y}$

  3. $y-\dfrac{1}{y}$

  4. $\dfrac{y}{1-y}$


Correct Option: D