Predicting What Comes Next: Sequences and Progressions | Exercise 8.2

Question 3

Find the nthn^{\text{th}} term of the AP: 11, 8, 5, 2 ... Write the recursive rule for this AP.

Check your answer with HomiSolve it yourself, then let Homi check your steps and spot mistakes.
Solution

Let's find the nthn^{\text{th}} term and the recursive rule for this AP.

Step 1 — Find the nthn^{\text{th}} term

First, we find the first term. Then, we find the common difference. The first term is a=11a = \textbf{11}. The common difference dd is the difference between consecutive terms.

d=811d = 8 - 11

3\boxed{-3}

Now, we use the formula for the nthn^{\text{th}} term of an AP. The formula is tn=a+(n1)dt_n = a + (n-1)d. Let's substitute the values of aa and dd.

tn=11+(n1)(3)t_n = 11 + (n-1)(-3)

=113n+3= 11 - 3n + 3

=143n= 14 - 3n

tn=143n\boxed{t_n = 14 - 3n}

Diagram 1

Step 2 — Write the recursive rule

A recursive rule defines a term using the previous term. We need the first term. We also need the common difference. The first term is t1=11t_1 = \textbf{11}. Each term is found by adding the common difference to the previous term.

tn=tn1+dt_n = t_{n-1} + d

tn=tn13t_n = t_{n-1} - 3

t1=11,tn=tn13 for n2\boxed{t_1 = 11, \quad t_n = t_{n-1} - 3 \text{ for } n \ge 2}

Answer

(i) The nthn^{\text{th}} term is 143n14 - 3n. (ii) The recursive rule is t1=11t_1 = 11, tn=tn13t_n = t_{n-1} - 3 for n2n \ge 2.

More questions in Exercise 8.2

Q1

Find the 10th10^{\text{th}} and 26th26^{\text{th}} terms of the AP: 3, 8, 13, 18, ....

Q2

Which term of the AP : 21, 18, 15, ... is 81-81? Also, is 0 a term of this AP? Give reasons for your answer.

Q3

Find the nthn^{\text{th}} term of the AP: 11, 8, 5, 2 ... Write the recursive rule for this AP.

Q4

An AP consists of 50 terms in which the 3rd3^{\text{rd}} term is 12 and the last term is 106. Find the 29th29^{\text{th}} term.

(Hint: If 'a' is the first term and 'd' the common difference, then we arrive at the equations a+2d=12a + 2d = 12 and a+49d=106a + 49d = 106. Solve this pair of linear equations for 'a' and 'd'.)

Q5

How many 2-digit numbers are divisible by 3? What is the sum of all these 2-digit numbers?

Q6

Harish started work at an annual salary of ₹5,00,000 and received an increment of ₹20,000 each year. After how many years did his income reach ₹7,00,000?

Q7

A child arranges marbles in rows so that the first row has 1 marble, the second has 2 marbles, the third has 3, and so on up to 25 rows. How many marbles does the child use in all?

← Back to Predicting What Comes Next: Sequences and Progressions