Which term?

Algebra Level 3

Consider an arithmetic progression:- 5 8 , 17 8 , 29 8 , 41 8 , \dfrac{5}{8}, \dfrac{17}{8}, \dfrac{29}{8}, \dfrac{41}{8}, \cdots . How many terms of this AP are needed to give the sum of the terms as 295 4 \dfrac{295}{4} ?


The answer is 10.

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2 solutions

Sabhrant Sachan
May 26, 2016

I totally Like your Solution @Ashish Siva , but we can Reduce the calculation .

We have an A.P of Common Difference ( d ) = 17 8 5 8 = 12 8 (d) = \dfrac{17}{8}-\dfrac58=\dfrac{12}8 and First Term ( a ) = 5 8 (a) = \dfrac{5}8 . Let the Required number of terms be n n

S n = 295 4 = n 2 ( 5 4 + ( n 1 ) 6 4 ) 590 = n ( 5 + 6 n 6 ) n ( 6 n 1 ) = 590 n ( 6 n 1 ) = 10 ( 6 × 10 1 ) n = 10 S_n=\dfrac{295}{4}=\dfrac{n}{2} \left(\dfrac54+(n-1)\dfrac64\right) \\ 590=n(5+6n-6) \\ n(6n-1)=590 \\ n(6n-1)=10(6\times10-1) \\ \boxed{n = 10 }

Since d > 0 d>0 , the quadratic will only give us only 1 1 +ve Solution

I thought that it would be lengthy so I posted an alternative solution plz do have a look. Btw nice solution (+1)

Ashish Menon - 5 years ago
Ashish Menon
May 26, 2016

Here we see that the first term of the AP(a) is 5 8 \dfrac{5}{8} and the common difference(d) = 17 5 8 = 12 8 = 3 2 \dfrac{17 - 5}{8} = \dfrac{12}{8} = \dfrac{3}{2} . Let the number of terms required be n n .

S n = n 2 × ( 2 a + ( n 1 ) d ) 295 4 = n 2 × ( 2 × 5 8 + ( n 1 ) × 3 2 ) 295 4 = n 2 × ( 5 4 + 3 n 3 2 ) 295 4 = n 2 × 5 + 6 n 6 4 295 4 = 6 n 2 n 8 295 × 8 4 = 6 n 2 n 590 = 6 n 2 n 6 n 2 n 590 = 0 S_n = \dfrac{n}{2} × \left(2a + (n - 1)d\right)\\ \dfrac{295}{4} = \dfrac{n}{2} × \left(2×\dfrac{5}{8} + (n - 1)×\dfrac{3}{2}\right)\\ \dfrac{295}{4} = \dfrac{n}{2} × \left(\dfrac{5}{4} + \dfrac{3n - 3}{2}\right)\\ \dfrac{295}{4} = \dfrac{n}{2} × \dfrac{5 + 6n - 6}{4}\\ \dfrac{295}{4} = \dfrac{6n^2 - n}{8}\\ \dfrac{295 × 8}{4} = 6n^2 - n\\ 590 = 6n^2 - n\\ 6n^2 - n - 590 = 0

Applying quadratic formula:-
n = ( 1 ) ± ( 1 ) 2 4 × 6 × 590 2 × 6 n = 1 ± 1 + 14160 12 n = 1 ± 14161 12 n = 1 ± 119 12 n = 1 + 119 12 ( or ) 1 119 12 n = 120 12 ( or ) = 118 12 n = 10 ( or ) 59 6 n = \dfrac{ -(-1) \pm \sqrt{{(-1)}^2 -4×6×-590}}{2×6}\\ \\ n = \dfrac{1 \pm \sqrt{1 + 14160}}{12}\\ \\ n = \dfrac{1 \pm \sqrt{14161}}{12}\\ \\ n = \dfrac{1 \pm 119}{12}\\ \\ n = \dfrac{1 + 119}{12} \left(\text{or}\right) \dfrac{1 - 119}{12}\\ \\ n = \dfrac{120}{12} \left(\text{or}\right) = \dfrac{-118}{12}\\ \\ n = 10 \left(\text{or}\right) \dfrac{-59}{6}

But, number of terms can never be negative. So, n 59 6 n \neq \dfrac{-59}{6} . So, n = 10 n = \color{#69047E}{\boxed{10}} .


Alternatively:-
Observe that the numerators of the terms are in AP with first term(a) = 5 5 and common difference 12 12 . Why? Because the actual common difference 3 2 \dfrac{3}{2} can be written as 12 8 \dfrac{12}{8} . So denominator remains the same but numerator increases by 12 12 .

The sum given whihc is 295 4 \dfrac{295}{4} can be written as 590 8 \dfrac{590}{8} . So, now we just have to find how much terms of the AP 5 , 17 , 29 , 41 , 5, 17, 29, 41, \cdots give the sum as 590 590 . Let m m terms be required.
S m = m 2 × ( 2 a + ( m 1 ) d ) 590 = m 2 × ( 2 × 5 + ( m 1 ) × 12 ) 590 = m 2 × 2 ( 5 + 6 ( m 1 ) ) 590 = m ( 5 + 6 m 6 ) 590 = 6 m 2 m 6 m 2 m 590 = 0 6 m 2 60 m + 59 m 590 = 0 6 m ( m 10 ) + 59 ( m 10 ) = 0 ( m 10 ) ( 6 m + 59 ) = 0 m 10 = 0 ( or ) 6 m + 59 = 0 n = 10 ( or ) 59 6 S_m = \dfrac{m}{2} × \left(2a + (m - 1)d\right)\\ 590 = \dfrac{m}{2} × \left(2×5 + (m - 1)×12\right)\\ 590 = \dfrac{m}{2} × 2\left(5 + 6(m - 1)\right)\\ 590 = m\left(5 + 6m - 6\right)\\ 590 = 6m^2 - m\\ 6m^2 - m - 590 = 0\\ 6m^2 - 60m + 59m - 590 = 0\\ 6m\left(m - 10\right) + 59\left(m - 10\right) = 0\\ \left(m - 10\right)\left(6m + 59\right) = 0\\ m - 10 = 0\left(\text{or}\right) 6m + 59 = 0\\ n = 10 \left(\text{or}\right) \dfrac{-59}{6}

But, number of terms can never be negative. So, n 59 6 n \neq \dfrac{-59}{6} . So, n = 10 n = \color{#69047E}{\boxed{10}} .

There is a Typo is the alternative answer

Sabhrant Sachan - 5 years ago

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Oops I corrected it. It sometimes take ages to track down a minor mistake in LaTeX.

Ashish Menon - 5 years ago

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AP is the easiest Math topic.

Abhiram Rao - 5 years ago

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@Abhiram Rao Correct and it is also the most beautiful one.

Ashish Menon - 5 years ago

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