Simple but tricky

Algebra Level 4

Given that i = 1 25 a i = 725 \displaystyle \sum_{i=1}^{25} a_i=725 , where a 1 , a 2 , a 3 , a_1,a_2,a_3,\ldots are in an arithmetic progression . Then find the value of r = 0 12 a 2 r + 1 \displaystyle \sum_{r=0}^{12} a_{2r+1} .

None of these choices 410 310 315 363 361 377 398

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2 solutions

Let the first term and common difference be a = a 1 a=a_1 and d d respectively. Then, we have:

i = 1 25 a i = 25 ( 2 a + 24 d ) 2 = 725 a + 12 d = 29 \begin{aligned} \sum_{i=1}^{25} a_i = \frac{25(2a+24d)}{2} & = 725 \\ \Rightarrow a + 12d & = 29 \end{aligned}

Now we have:

r = 0 12 a 2 r + 1 = a 1 + a 3 + a 5 + . . . + a 25 = a + ( a + 2 d ) + ( a + 4 d ) + . . . + ( a + 24 d ) = 13 a + 2 × 12 ( 12 + 1 ) 2 d = 13 ( a + 12 d ) = 13 × 29 = 377 \begin{aligned} \sum_{r=0}^{12} a_{2r+1} & = a_1+a_3+a_5+...+a_{25} \\ & = a + (a+2d)+(a+4d)+...+(a+24d) \\ & = 13a + 2\times \frac{12(12+1)}{2}d \\ & = 13(a+12d) = 13 \times 29 = \boxed{377} \end{aligned}

Atul Shivam
Dec 4, 2015

a 1 + a 2 + . . . a 25 = 725 a_1+a_2+...a_{25}=725 25 2 ( a 1 + a 25 = 725 \frac{25}{2}(a_1+a_{25}=725 a 1 + a 25 = 58 a_1+a_{25}=58

Now we have to calculate a 1 + a 3 + . . . + a 25 a_1+a_3+...+a_{25} For this we can do ( a 1 + a 25 ) + ( a 3 + a 23 ) + . . . + ( a 11 + a 15 ) + a 13 (a_1+a_{25})+(a_3+a_{23})+...+(a_{11}+a_{15})+a_{13} Which can also be written as ( a 1 + a 25 ) + ( a 1 + a 25 ) + . . . + ( a 1 + a 25 ) + ( a 1 + a 25 ) 2 (a_1+a_{25})+(a_1+a_{25})+...+(a_1+a_{25})+\frac{(a_1+a_{25})}{2}

Which is 13 2 ( a 1 + a 25 ) = 13 2 58 = 377 \frac{13}{2}(a_1+a_{25})=\frac{13}{2}58=377 So the correct answer is 377 \boxed{377}

In your sum in the problem, you ask to find a 3 + a 5 + + a 25 a_3+a_5+\cdots+a_{25} , while in your solution you find a 1 + a 3 + a 25 a_1+a_3+\cdots a_{25} . I think that you meant to start from r = 0 r=0 .

Alan Enrique Ontiveros Salazar - 5 years, 6 months ago

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Yup I got it now from @cheo-Seong Cheong and hence edited the problem accordingly!!!

Atul Shivam - 5 years, 6 months ago

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