Question

For an A.P if a=1 d=4 what is the value of n if tn=81.
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Answer 1

Step-by-step explanation:

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Answer 2

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Q1) For an AP if a=1,d=4.what is the value of n if tn=81

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$\\ \quad\implies\displaystyle\tt{t_n=a+(n-1)d}$

$\\ \quad\implies\displaystyle\tt{81=1+(n-1) \times 4}$

$\\ \quad\implies\displaystyle\tt{81-1=(n-1)4}$

$\\ \quad\implies\displaystyle\tt{80=4n-4}$

$\\ \quad\implies\displaystyle\tt{80-4n+4=0}$

$\\ \quad\implies\displaystyle\tt{84=4n}$

$\\ \quad\implies\displaystyle\tt{4n=84}$

$\\ \quad\implies\displaystyle\tt{n=\dfrac{84}{4}}$

$\\ \quad\implies\displaystyle\tt{n=\cancel\dfrac{84}{4}=21}$

$\\ \quad\implies\underline{\displaystyle\tt{n=21}}$

$\\ \quad\boxed{\displaystyle\sf{number \: of \: terms =21}}$

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