Predicting What Comes Next: Sequences and Progressions | Exercise 8.2

Question 1

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

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Solution

We will use the formula for the nthn^{\text{th}} term of an arithmetic progression.

Step 1 — Find the first term and common difference

First, let's identify the first term. The first term of the AP is 3. Let's find the common difference. We subtract any term from its next term.

d=83d = 8 - 3

d=5\boxed{d = 5}

Diagram 1

Step 2 — Calculate the 10th term

The formula for the nthn^{\text{th}} term is tn=a+(n1)dt_n = a + (n-1)d. Here, a=3a = \mathbf{3} and d=5d = \mathbf{5}. We need to find the 10th10^{\text{th}} term, so n=10n = \mathbf{10}. Let's substitute these values into the formula.

t10=3+(101)×5t_{10} = 3 + (10 - 1) \times 5

=3+(9)×5= 3 + (9) \times 5

=3+45= 3 + 45

t10=48\boxed{t_{10} = 48}

Step 3 — Calculate the 26th term

We use the same formula. Again, a=3a = \mathbf{3} and d=5d = \mathbf{5}. Now we need the 26th26^{\text{th}} term, so n=26n = \mathbf{26}. Let's put these values into the formula.

t26=3+(261)×5t_{26} = 3 + (26 - 1) \times 5

=3+(25)×5= 3 + (25) \times 5

=3+125= 3 + 125

t26=128\boxed{t_{26} = 128}

Answer

(i) The 10th10^{\text{th}} term is 48. (ii) The 26th26^{\text{th}} term is 128.

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?

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