What a familiar progression!

Algebra Level 2

1 , ______ , ______ , ______ , ______ , 2017 1 \; , \; \text{\_\_\_\_\_\_} \; , \; \text{\_\_\_\_\_\_} \; , \; \text{\_\_\_\_\_\_} \; , \; \text{\_\_\_\_\_\_} \; , \; 2017

Brian has written up 6 numbers, where each subsequent number is larger than the one before it, and he plans to add up these numbers.

However, due to his terrible handwriting, he is unable to decipher all but the first and last numbers, as illustrated above.

He does, however, recall that the differences between two consecutive numbers are all the same. Can you help him find the sum of these 6 numbers?


The answer is 6054.

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5 solutions

Eric Hedberg
Feb 1, 2017

Take for instance the number set 10, 20, 30, 40, 50, 60. There are many ways to find the sum, but since the difference between each number is the same, you can add up the opposite numbers (first with last, second with second to last, third with third to last) and they'd all have the same sum. Here it would be 70 (10 + 60), and since there are three of those like sets, you can multiply that by 3 to get the sum. In this problem here, 1 + 2017 = 2018. 2018 x 3 = 6054.

Nice observation! We can find the sum without actually knowing what numbers fill the blanks, by forming three pairs of numbers, each of which has the same sum.

We could form three pairs because there are an even number of terms in the sequence. How would we go about the problem if there were 5 terms in the sequence?

Pranshu Gaba - 4 years, 4 months ago

Relevant wiki: Arithmetic and Geometric Progressions Problem Solving

Since the difference between each consecutive numbers are the same, the given sequence is an arithmetic progression whose sum is given by n 2 ( a + l ) , \frac{n}{2}(a+l), where a a is 1st term and l l is last term. So we do not need the middle terms at all and the answer is 6054.

Ah, it's useful to know that there are 2 arithmetic progression sum formulas (which are actually equivalent to each other):

S n = n 2 ( a + l ) = n 2 [ 2 a + ( n 1 ) d ] . S_ n = \dfrac n2 (a + l) = \dfrac n2 [ 2a + (n-1) d ] .

Pi Han Goh - 4 years, 4 months ago
Deva Craig
Feb 18, 2017

Since the differences between two consecutive numbers in the sequence are all the same, the sequence is an arithmetic sequence.

First, you will start with the formula:

a n a_{n} = a i a_{i} + d(n - 1)

a n a_{n} = nth term in the sequence a i a_{i} = First term in the sequence n = number of terms in the sequence d = the common difference

Essentially, in this problem, we're looking for d .

First, we plug in what we already know about the problem.

a n a_{n} = 2017 a i a_{i} = 1 n = 6 d = ?

Next, we plug in all of the known information in our formula:

2017 = 1 + d(6 - 1)

Next, we solve for d:

  1. 2017 = 1 + d(6-1)
  2. 2017 = 1 + d(5)
  3. 2017 - 1 = 1 + d(5) - 1
  4. 2016/5 = d(5)/5
  5. 403.2

Now that we have our common difference d , we can find the missing four terms by continually adding 403.2 onto term n-1. So we get the following sequence:

1, 404.2, 807.4, 1210.6, 1613.8, 2017.

We then find the sum of all of the terms:

1 + 404.2 + 807.4 + 1210.6 + 1613.8 + 2017 = 6054 \boxed{6054}

This is a complete solution. Note that we don't actually have to find "d" to get the desired answer (as shown in other solutions).

Pi Han Goh - 4 years, 3 months ago
Emmett Jesrani
Feb 1, 2017

Using the same idea as the famous 1-100 sum, we take pairs which are equidistant from the middle (i.e. 1 and 2017) and add them. In this case, the sums are 1+2017=2018, so 2018*3=6054 is our final answer!

Jesse Nieminen
Feb 1, 2017

Since the differences between two consecutive numbers are all the same, the numbers form an arithmetic progression .

Now, let a a be the initial term and d d be the constant difference between two consecutive numbers.

Now, clearly:

a = 1 and a + 5 d = 2017 a = 1 \ \text{and} \ a + 5d = 2017

And the sum of the numbers is equal to 6 a + 15 d = 3 a + 3 a + 15 d = 3 ( a ) + 3 ( a + 5 d ) = 3 1 + 3 2017 = 6054 6a + 15d = 3a + 3a + 15d = 3(a) + 3(a + 5d) = 3\cdot1 + 3\cdot2017 = \boxed{6054} which is the answer.

Clear and concise algebraic solution! Thanks!!

Pi Han Goh - 4 years, 4 months ago

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