Answer
Verified
399k+ views
Hint: Break the terms of the given series as the sum of exponents of 3 and 4, that is, \[\left( {{3}^{0}}+{{4}^{0}} \right)+\left( {{3}^{1}}+{{4}^{1}} \right)x+\left( {{3}^{2}}+{{4}^{2}} \right){{x}^{2}}+\left( {{3}^{3}}+{{4}^{3}} \right){{x}^{3}}+......\]. Start from the power ‘0’ of 3 and 4 and use the formula: ${{T}_{n}}=a{{r}^{n-1}}$ to determine the ${{n}^{th}}$ term of the G.P series obtained. Here, ${{T}_{n}}$ is the ${{n}^{th}}$ term, ‘a’ is the first term and ‘r’ is the common ratio of the series. To determine the sum of these terms use the relation: ${{S}_{n}}=\dfrac{a\left( {{r}^{n}}-1 \right)}{r-1}$.
Complete step-by-step answer:
In this question geometric progression will be used. In Mathematics, a G.P. also known as geometric sequence is a sequence of numbers where each term after its predecessor is obtained by multiplying the previous term with a fixed non-zero number known as the common ratio of the G.P.
For example: \[2,\text{ }4,\text{ }8,\text{ }16,\text{ }32,\text{ }.........\] is a G.P. with common ratio $2$.
Now, we come to the question. We have been given the series $2+7x+25{{x}^{2}}+91{{x}^{3}}+.........$
This can be written as:
\[\begin{align}
& \left( {{3}^{0}}+{{4}^{0}} \right)+\left( {{3}^{1}}+{{4}^{1}} \right)x+\left( {{3}^{2}}+{{4}^{2}} \right){{x}^{2}}+\left( {{3}^{3}}+{{4}^{3}} \right){{x}^{3}}+...... \\
& =\left( {{3}^{0}}{{x}^{0}}+{{3}^{1}}{{x}^{1}}+{{3}^{2}}{{x}^{2}}+{{3}^{3}}{{x}^{3}}+..... \right)+\left( {{4}^{0}}{{x}^{0}}+{{4}^{1}}{{x}^{1}}+{{4}^{2}}{{x}^{2}}+{{4}^{3}}{{x}^{3}}+..... \right) \\
& =\left( 1+{{3}^{1}}{{x}^{1}}+{{3}^{2}}{{x}^{2}}+{{3}^{3}}{{x}^{3}}+..... \right)+\left( 1+{{4}^{1}}{{x}^{1}}+{{4}^{2}}{{x}^{2}}+{{4}^{3}}{{x}^{3}}+..... \right) \\
\end{align}\]
Now, the pattern is clear to us. Clearly, we can see that for \[\left( 1+{{3}^{1}}{{x}^{1}}+{{3}^{2}}{{x}^{2}}+{{3}^{3}}{{x}^{3}}+..... \right)\],
a = first term = 1 and r = common ratio = 3x.
Now, for \[\left( 1+{{4}^{1}}{{x}^{1}}+{{4}^{2}}{{x}^{2}}+{{4}^{3}}{{x}^{3}}+..... \right)\],
a = first term = 1 and r = common ratio = 4x.
Therefore, applying the formula for ${{n}^{th}}$ term of the G.P, we get,
$\begin{align}
& {{T}_{n}}=a{{r}^{n-1}} \\
& \Rightarrow {{T}_{n}}=1\times {{\left( 3x \right)}^{n-1}}+1\times {{\left( 4x \right)}^{n-1}} \\
& \Rightarrow {{T}_{n}}={{\left( 3x \right)}^{n-1}}+{{\left( 4x \right)}^{n-1}} \\
& \Rightarrow {{T}_{n}}={{\left( x \right)}^{n-1}}\left( {{3}^{n-1}}+{{4}^{n-1}} \right) \\
\end{align}$
We know that sum of n terms of a G.P is ${{S}_{n}}=\dfrac{a\left( {{r}^{n}}-1 \right)}{r-1}$. Therefore, the sum of the given series is:
$\begin{align}
& {{S}_{n}}=\dfrac{1\times \left( {{3}^{n}}{{x}^{n}}-1 \right)}{3x-1}+\dfrac{1\times \left( {{4}^{n}}{{x}^{n}}-1 \right)}{4x-1} \\
& \therefore {{S}_{n}}\text{ }=\dfrac{{{\left( 3x \right)}^{n}}-1}{3x-1}+\dfrac{{{\left( 4x \right)}^{n}}-1}{4x-1} \\
\end{align}$
Note:We can check our answer by substituting the value of ‘n’ in the expression of sum of n terms. Also, as you can see that we have broken the terms to extract a pattern, otherwise it would have been very difficult to solve the problem.
Complete step-by-step answer:
In this question geometric progression will be used. In Mathematics, a G.P. also known as geometric sequence is a sequence of numbers where each term after its predecessor is obtained by multiplying the previous term with a fixed non-zero number known as the common ratio of the G.P.
For example: \[2,\text{ }4,\text{ }8,\text{ }16,\text{ }32,\text{ }.........\] is a G.P. with common ratio $2$.
Now, we come to the question. We have been given the series $2+7x+25{{x}^{2}}+91{{x}^{3}}+.........$
This can be written as:
\[\begin{align}
& \left( {{3}^{0}}+{{4}^{0}} \right)+\left( {{3}^{1}}+{{4}^{1}} \right)x+\left( {{3}^{2}}+{{4}^{2}} \right){{x}^{2}}+\left( {{3}^{3}}+{{4}^{3}} \right){{x}^{3}}+...... \\
& =\left( {{3}^{0}}{{x}^{0}}+{{3}^{1}}{{x}^{1}}+{{3}^{2}}{{x}^{2}}+{{3}^{3}}{{x}^{3}}+..... \right)+\left( {{4}^{0}}{{x}^{0}}+{{4}^{1}}{{x}^{1}}+{{4}^{2}}{{x}^{2}}+{{4}^{3}}{{x}^{3}}+..... \right) \\
& =\left( 1+{{3}^{1}}{{x}^{1}}+{{3}^{2}}{{x}^{2}}+{{3}^{3}}{{x}^{3}}+..... \right)+\left( 1+{{4}^{1}}{{x}^{1}}+{{4}^{2}}{{x}^{2}}+{{4}^{3}}{{x}^{3}}+..... \right) \\
\end{align}\]
Now, the pattern is clear to us. Clearly, we can see that for \[\left( 1+{{3}^{1}}{{x}^{1}}+{{3}^{2}}{{x}^{2}}+{{3}^{3}}{{x}^{3}}+..... \right)\],
a = first term = 1 and r = common ratio = 3x.
Now, for \[\left( 1+{{4}^{1}}{{x}^{1}}+{{4}^{2}}{{x}^{2}}+{{4}^{3}}{{x}^{3}}+..... \right)\],
a = first term = 1 and r = common ratio = 4x.
Therefore, applying the formula for ${{n}^{th}}$ term of the G.P, we get,
$\begin{align}
& {{T}_{n}}=a{{r}^{n-1}} \\
& \Rightarrow {{T}_{n}}=1\times {{\left( 3x \right)}^{n-1}}+1\times {{\left( 4x \right)}^{n-1}} \\
& \Rightarrow {{T}_{n}}={{\left( 3x \right)}^{n-1}}+{{\left( 4x \right)}^{n-1}} \\
& \Rightarrow {{T}_{n}}={{\left( x \right)}^{n-1}}\left( {{3}^{n-1}}+{{4}^{n-1}} \right) \\
\end{align}$
We know that sum of n terms of a G.P is ${{S}_{n}}=\dfrac{a\left( {{r}^{n}}-1 \right)}{r-1}$. Therefore, the sum of the given series is:
$\begin{align}
& {{S}_{n}}=\dfrac{1\times \left( {{3}^{n}}{{x}^{n}}-1 \right)}{3x-1}+\dfrac{1\times \left( {{4}^{n}}{{x}^{n}}-1 \right)}{4x-1} \\
& \therefore {{S}_{n}}\text{ }=\dfrac{{{\left( 3x \right)}^{n}}-1}{3x-1}+\dfrac{{{\left( 4x \right)}^{n}}-1}{4x-1} \\
\end{align}$
Note:We can check our answer by substituting the value of ‘n’ in the expression of sum of n terms. Also, as you can see that we have broken the terms to extract a pattern, otherwise it would have been very difficult to solve the problem.
Recently Updated Pages
Basicity of sulphurous acid and sulphuric acid are
Assertion The resistivity of a semiconductor increases class 13 physics CBSE
The Equation xxx + 2 is Satisfied when x is Equal to Class 10 Maths
What is the stopping potential when the metal with class 12 physics JEE_Main
The momentum of a photon is 2 times 10 16gm cmsec Its class 12 physics JEE_Main
Using the following information to help you answer class 12 chemistry CBSE
Trending doubts
Difference between Prokaryotic cell and Eukaryotic class 11 biology CBSE
Fill the blanks with the suitable prepositions 1 The class 9 english CBSE
Two charges are placed at a certain distance apart class 12 physics CBSE
Difference Between Plant Cell and Animal Cell
What organs are located on the left side of your body class 11 biology CBSE
Change the following sentences into negative and interrogative class 10 english CBSE
The planet nearest to earth is A Mercury B Venus C class 6 social science CBSE
The Equation xxx + 2 is Satisfied when x is Equal to Class 10 Maths
What is BLO What is the full form of BLO class 8 social science CBSE