Question

3. Consider the sequence,-8, -5, -2, 1, ...
a) Determine the explicit formula for the general term, 1,, of this sequence in simplified
form. (2 marks)
b) Use this formula to determine the value of t20. (1 mark)
c) Algebraically determine which term has a value of 40. (1 mark)

Answer 1

Answer:

a) $a_n=3\,n-11$

b) $a_{20}=49$

c) term number 17 is the one that gives a value of 40

Step-by-step explanation:

a)

The sequence seems to be arithmetic, and with common difference d = 3.

Notice that when you add 3 units to the first term (-80, you get :

-8 + 3 = -5

and then -5 + 3 = -2 which is the third term.

Then, we can use the general form for the nth term of an arithmetic sequence to find its simplified form:

$a_n=a_1+(n-1)\,d$

That in our case would give:

$a_n=-8+(n-1)\,(3)\\a_n=-8+3\,n-3\\a_n=3n-11$

b)

Therefore, the term number 20 can be calculated from it:

$a_{20}=3\,(20)-11=60-11=49$

c) in order to find which term renders 20, we use the general form we found in step a):

$a_n=3\,n-11\\40=3\,n-11\\40+11=3\,n\\51=3\,n\\n=\frac{51}{3} =17$

so term number 17 is the one that renders a value of 40