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# In a certain sequence, the term an is defined by the formula an = an –

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Joined: 02 Sep 2009
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In a certain sequence, the term an is defined by the formula an = an –  [#permalink]

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29 Jul 2018, 23:39
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In a certain sequence, the term an is defined by the formula $$a_n = a_{n – 1} + 5$$ for each integer n ≥ 2. If $$a_1 = 1$$, what is the sum of the first 75 terms of this sequence?

(A) 10,150
(B) 11,375
(C) 12,500
(D) 13,950
(E) 15,375

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Re: In a certain sequence, the term an is defined by the formula an = an –  [#permalink]

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30 Jul 2018, 04:43
Bunuel wrote:
In a certain sequence, the term an is defined by the formula $$a_n = a_{n – 1} + 5$$ for each integer n ≥ 2. If $$a_1 = 1$$, what is the sum of the first 75 terms of this sequence?

(A) 10,150
(B) 11,375
(C) 12,500
(D) 13,950
(E) 15,375

It's an AP as each term is 5 more than the previous term..

1) a_1=1 so a_75=1+(75-1)*5=1+370=371..
Sum is average of the first and last number *#=(1+371)/2*75=372/2*75=186*75=13950

2) choices..
AP means the sum would be multiple of # so multiple of 75 that means 3..
Only D and E are multiple of 3..
And D is even and E is odd..
So can we find out if sum is odd or even
Since we are adding odd number one after another alternate will be odd..
74 will have 74/2=37 odd numbers and 75th is also odd..
So total odd numbers =37+1=38 and SUM of 39 odds will be even
So our answer is even

D
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1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html
3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html

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In a certain sequence, the term an is defined by the formula an = an –  [#permalink]

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30 Jul 2018, 17:46
1
Bunuel wrote:
In a certain sequence, the term $$a_n$$ is defined by the formula $$a_n = a_{n – 1} + 5$$ for each integer n ≥ 2. If $$a_1 = 1$$, what is the sum of the first 75 terms of this sequence?

(A) 10,150
(B) 11,375
(C) 12,500
(D) 13,950
(E) 15,375

$$a_n$$ is defined by the formula $$a_n = a_{n – 1} + 5$$ for each integer n ≥ 2. If $$a_1 = 1$$,

$$A_1=1 ------>A_1=1$$
$$A_2=A_1+5=6->A_2=1+5$$
$$A_3=A_2+5=11->A_3=1+5+5$$$$A_4=A_3+5=16->A_4 =1+5+5+5$$

The sequence on the right just helps to show that this is an arithmetic series with a common difference of 5 and a start term of 1, that is
$$A_n=1+(n-1)5$$

$$A_2$$ is 1 + one 5
$$A_3$$ is 1 + two 5s
$$A_4$$ is 1 + three 5s
$$A_5$$ will be 1 + four 5s
$$A_{6}$$ = 1 + five 5s

Each term starts with $$A_1=1$$
Each term has a common difference of 5 (increases by 5) -- BUT
Each Term$$_{n}$$ has $$(n-1)$$ 5s.
Each term's number of 5s is one fewer than itself.
$$A_{75}$$ will have $$(n-1) = 74$$ fives

$$A_n=1+(n-1)5$$: sequence rule from above

$$A_{75}=(1+(75-1)*5)=$$
$$A_{75}=(1+(74*5))$$
$$A_{75}=(1+370)=371$$

Sum of the sequence: (average)*(# of terms)

Average in an evenly spaced sequence is $$\frac{(First+ Last)}{2}$$

# of terms, given, is 75

Sum: $$\frac{(First+ Last)}{2}*75$$

Sum = $$(\frac{1+371}{2}*75)=(186*75)$$
Double and halve
Sum = $$(93*150)=$$
Split the 93
Sum of sequence =$$(90*150+3*150)=(13,500+450)= 13,950$$

For an excellent post on sequences that is easy to follow, see this topic thread on GMAT Club, and scroll down to the post by benjiboo

See also the Ultimate GMAT Quantitative Megathread , 12. Sequences
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Re: In a certain sequence, the term an is defined by the formula an = an –  [#permalink]

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30 Jul 2018, 19:53
Bunuel wrote:
In a certain sequence, the term an is defined by the formula $$a_n = a_{n – 1} + 5$$ for each integer n ≥ 2. If $$a_1 = 1$$, what is the sum of the first 75 terms of this sequence?

(A) 10,150
(B) 11,375
(C) 12,500
(D) 13,950
(E) 15,375

One of the formulas for Sum of first n members of arithmetic progression is:

S = (2a{1} + d*(n-1))/2 * n

(The othe one is (a{1}+a{n})/2 * n)

a{1} = 1, d = 5 - substitute these numbers and will get option D
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Re: In a certain sequence, the term an is defined by the formula an = an –  [#permalink]

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06 Aug 2018, 04:46
DharLog wrote:
Bunuel wrote:
In a certain sequence, the term an is defined by the formula $$a_n = a_{n – 1} + 5$$ for each integer n ≥ 2. If $$a_1 = 1$$, what is the sum of the first 75 terms of this sequence?

(A) 10,150
(B) 11,375
(C) 12,500
(D) 13,950
(E) 15,375

One of the formulas for Sum of first n members of arithmetic progression is:

S = (2a{1} + d*(n-1))/2 * n

(The othe one is (a{1}+a{n})/2 * n)

a{1} = 1, d = 5 - substitute these numbers and will get option D

Would y explain ? plz
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Re: In a certain sequence, the term an is defined by the formula an = an –  [#permalink]

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06 Aug 2018, 13:27
1
vivapopo wrote:
DharLog wrote:
Bunuel wrote:
In a certain sequence, the term an is defined by the formula $$a_n = a_{n – 1} + 5$$ for each integer n ≥ 2. If $$a_1 = 1$$, what is the sum of the first 75 terms of this sequence?

(A) 10,150
(B) 11,375
(C) 12,500
(D) 13,950
(E) 15,375

One of the formulas for Sum of first n members of arithmetic progression is:

S = (2a{1} + d*(n-1))/2 * n

(The othe one is (a{1}+a{n})/2 * n)

a{1} = 1, d = 5 - substitute these numbers and will get option D

Would y explain ? plz

Sure!
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Re: In a certain sequence, the term an is defined by the formula an = an –  [#permalink]

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10 Aug 2018, 17:48
Bunuel wrote:
In a certain sequence, the term an is defined by the formula $$a_n = a_{n – 1} + 5$$ for each integer n ≥ 2. If $$a_1 = 1$$, what is the sum of the first 75 terms of this sequence?

(A) 10,150
(B) 11,375
(C) 12,500
(D) 13,950
(E) 15,375

We see that a1 = 1, a2 = 1 + 5 = 6, a3 = 1 + 2 x 5 = 11, …, and a75 = 1 + 74 x 5 = 371.

We will use the formula sum = average x quantity. Recall that the average of an evenly spaced set is (smallest element + largest element)/2; thus, average = (371 + 1)/2 = 186. Thus, the sum is 186 x 75 = 13,950.

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Re: In a certain sequence, the term an is defined by the formula an = an – &nbs [#permalink] 10 Aug 2018, 17:48
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# In a certain sequence, the term an is defined by the formula an = an –

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