Predicting What Comes Next: Sequences and Progressions | Exercise 8.1

Question 2

Find the 10th10^{\text{th}} and 15th15^{\text{th}} terms of the sequence tn=5n3t_n = 5n - 3 for n1n \ge 1.

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Solution
Understand the Question
  • The sequence is defined by the general formula tn=5n3t_n = 5n - 3, where tnt_n represents the nthn^{\text{th}} term.
  • To find any specific term of the sequence, substitute the given term number nn directly into the formula.
  • We need to evaluate the formula for n=10n = 10 and n=15n = 15.

Step 1 · Find the 10th10^{\text{th}} Term

Substitute n=10n = 10 into the formula tn=5n3t_n = 5n - 3Diagram 1

t10=5(10)3=503=47\begin{aligned} t_{10} &= 5(10) - 3 \\ &= 50 - 3 \\ &= 47 \end{aligned}

Step 2 · Find the 15th15^{\text{th}} Term

Substitute n=15n = 15 into the formula tn=5n3t_n = 5n - 3

t15=5(15)3=753=72\begin{aligned} t_{15} &= 5(15) - 3 \\ &= 75 - 3 \\ &= 72 \end{aligned}
Answer

The 10th10^{\text{th}} term is 4747 and the 15th15^{\text{th}} term is 7272.

Common Mistakes
  • Order of Operations: Incorrectly subtracting before multiplying, e.g., computing 5(103)=355(10 - 3) = 35 instead of 5(10)3=475(10) - 3 = 47.
  • Index Confusion: Substituting the desired term number into tnt_n instead of replacing nn in the expression.

More questions in Exercise 8.1

Q1

Find the first five terms of the sequence in which the nthn^{\text{th}} term is given by, for n1n \ge 1:

(i) tn=3n4t_n = 3n - 4

(ii) tn=25nt_n = 2 - 5n

(iii) tn=n22n+3t_n = n^2 - 2n + 3

Q2

Find the 10th10^{\text{th}} and 15th15^{\text{th}} terms of the sequence tn=5n3t_n = 5n - 3 for n1n \ge 1.

Q3

Determine whether 97 and 172 are terms of the sequence tn=5n3t_n = 5n - 3 for n1n \ge 1.

Q4

Which term of the sequence tn=5n3t_n = 5n - 3 for n1n \ge 1 is 607?

Q5

A sequence is given by the recursive rule t1=5t_1 = -5, tn+1=tn+3t_{n+1} = t_n + 3 for n1n \ge 1. Find the first five terms of the sequence. Is 5252 a term of this sequence? If so, which term is it?

Q6

Let T1=1T_1 = 1, T2=2T_2 = 2, T3=4T_3 = 4, and Tn=Tn1+Tn2+Tn3T_n = T_{n-1} + T_{n-2} + T_{n-3} for n4n \ge 4. Find T4T_4, T5T_5, T6T_6, T7T_7, and T8T_8.

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