ACT Math : Common Difference in Sequences

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #1 : Sequences

In the sequence 3, ____, ____, 24, what numbers can fill the two blanks so that consecutive terms differ by a common ratio?

 

Possible Answers:

10, 15

12, 16

9, 18

6, 12

10, 17

Correct answer:

6, 12

Explanation:

If the common ratio is r, then the sequence can be rewritten as 3, 3r, , . We see then that , which gives us that r=2. Therefore, the missing terms are 6 and 12.

 

 

 

Example Question #3 : Sequences

Find the common difference of the following sequence:

\(\displaystyle 4, 9, 14,19,24,29,...\)

Possible Answers:

\(\displaystyle 10\)

\(\displaystyle 5\)

\(\displaystyle 6\)

\(\displaystyle 3\)

Correct answer:

\(\displaystyle 5\)

Explanation:

The common difference is equal to

\(\displaystyle a_{n+1}\:-\:a_{n}\)

Plugging our values into this equation we can find the common difference.

\(\displaystyle 9-4=5\)

\(\displaystyle 14-9=5\)

\(\displaystyle \vdots\)

Therefore, in this case the common difference is \(\displaystyle 5\).

Example Question #1 : Common Difference In Sequences

Find the common difference of the following sequence:

\(\displaystyle 1,4,7,10,13,16,19,22,25,28,...\)

Possible Answers:

\(\displaystyle 4\)

\(\displaystyle 3\)

\(\displaystyle 6\)

\(\displaystyle 2\)

Correct answer:

\(\displaystyle 3\)

Explanation:

The common difference is equal to

\(\displaystyle a_{n+1}\:-\:a_{n}\).

Plugging in the values from this problem we get,

\(\displaystyle 4-1=3\)

\(\displaystyle 7-4=3\)

\(\displaystyle \vdots\)

Therefore, in this case the common difference is \(\displaystyle 3\).

Example Question #1 : How To Find The Common Difference In Sequences

Find the common difference of the following sequence:

\(\displaystyle 2,4,6,8,10,12,...\)

Possible Answers:

\(\displaystyle 2\)

\(\displaystyle 1\)

\(\displaystyle 4\)

\(\displaystyle 3\)

Correct answer:

\(\displaystyle 2\)

Explanation:

The common difference is equal to

\(\displaystyle a_{n+1}\:-\:a_{n}\).

Plugging in the values from this problem we get,

\(\displaystyle 4-2=2\)

\(\displaystyle 6-4=2\)

\(\displaystyle \vdots\)

Therefore, in this case the common difference is \(\displaystyle 2\).

Example Question #1 : How To Find The Common Difference In Sequences

The following is an arithmetic sequence. Find an explicit equation for it in terms of the common difference.

\(\displaystyle {\large} 3, 7, 11, 15, 19,...{}\)

Possible Answers:

\(\displaystyle {\large} a_n=3n+1\)

\(\displaystyle {\large} a_n=4n+2\)

\(\displaystyle {\large} a_n = 4n-1\)

\(\displaystyle {\large} a_n=3n\)

Correct answer:

\(\displaystyle {\large} a_n = 4n-1\)

Explanation:

Finding the common difference is fairly simple. We simply subtract the first term from the second. 7-3 = 4, so 4 is our common difference. So each term is going to be 4n plus something: 

\(\displaystyle {\large} a_n = 4n+x\)

We know the first term is 3, so we can plug in that to our equation.

\(\displaystyle {\large} 3=4+x\)

\(\displaystyle \large x=-1\)

So the explicit form of our arithmetic sequence is 

\(\displaystyle {\large} a_n=4n-1\).

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