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NO LINKS! PLEASE help me with this problem 3f​

NO LINKS! PLEASE help me with this problem 3f​-example-1
User Jeff Wang
by
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2 Answers

19 votes
19 votes

Answer:


\textsf{a)} \quad a_1=-6, \quad d=4

Explanation:

General formula of an arithmetic sequence


\boxed{s_n=s_1+(n-1)d}

where:

  • sₙ is the nth term.
  • s₁ is the first term.
  • d is the common difference between terms.
  • n is the position of the term.

Given terms:

  • s₅ = 10
  • s₈ = 22

Substitute the given terms into the formula to create two equations:


\begin{aligned}\implies s_5=s_1+(5-1)d&=10\\s_1+4d&=10\end{aligned}


\begin{aligned}\implies s_8=s_1+(8-1)d&=22\\s_1+7d&=22\end{aligned}

Subtract the first equation from the second equation to eliminate s₁:


\begin{array}{rrclcl}& s_1 &+ &7d &= &22\\-& (s_1 &+ &4d& =& 10)\\\cline{2-6}&&&3d &=& 12\end{array}

Solve for d:


\implies 3d=12


\implies (3d)/(3)=(12)/(3)


\implies d=4

Substitute the found value of d into one of the equations and solve for s₁:


\implies s_1+4(4)=10


\implies s_1+16=10


\implies s_1=-6

Therefore:

  • s₁ = -6
  • d = 4
16 votes
16 votes

Answer: Choice A


s_1 = -6, \ d = 4

==================================================

Work Shown:

a =
s_1 = first term

d = common difference

Plug n = 5 into the arithmetic sequence formula to get...


s_n = s_1 + d(n-1)\\\\s_5 = a + d(5-1)\\\\10 = a + 4d\\\\a+4d = 10\\\\a = 10-4d\\\\

Do similar steps for n = 8


s_n = s_1 + d(n-1)\\\\s_8 = a + d(8-1)\\\\22 = a + 7d\\\\a+7d = 22\\\\

Now plug in a = 10-4d and solve for d.


a+7d = 22\\\\10-4d+7d = 22\\\\10+3d = 22\\\\3d = 22-10\\\\3d = 12\\\\d = 12/3\\\\d = 4\\\\

The common difference is d = 4. This narrows the answer choices between A and D.

Use d = 4 to find 'a'.


a = 10-4d\\\\a = 10-4*4\\\\a = 10-16\\\\a = -6\\\\

Alternatively you could use the other equations that had 'a' and 'd' in them.

------------------

The first term is a = -6 and the common difference is d = 4

The arithmetic sequence is:

-6, -2, 2, 6, 10, 14, 18, 22, ...

The terms in bold represent the 5th and 8th terms respectively. They are highlighted to help confirm the answers.

It might help to make a table like this


\begin{array}c \cline{1-2}n & s_n\\\cline{1-2}1 & -6\\\cline{1-2}2 & -2\\\cline{1-2}3 & 2\\\cline{1-2}4 & 6\\\cline{1-2}5 & \boldsymbol{10}\\\cline{1-2}6 & 14\\\cline{1-2}7 & 18\\\cline{1-2}8 & \boldsymbol{22}\\\cline{1-2}\end{array}

or like this


\begin{array}c \cline{1-9}n & 1 & 2 & 3 & 4 & 5 & 6 & 7 & 8\\\cline{1-9}s_n & -6 & -2 & 2 & 6 & \boldsymbol{10} & 14 & 18 & \boldsymbol{22}\\\cline{1-9}\end{array}

Each time you need a new term, add on 4.

User Targumon
by
2.9k points