Exercise 2.5 - Chapter 2 - Algebra - 11th Business Maths Guide Samacheer Kalvi Solutions

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Exercise 2.5

Text Book Back Questions and Answers

By the principle of mathematical induction, prove the following:

Question 1.

Solution:

= 1
∴ P(1) is true.
Assume that P(n) is true n = k

⇒ P(k + 1) is true, whenever P(k) is true.
Hence, by the principle of mathematical induction P(n) is true for all n ∈ N.

Question 2.

Solution:
Let P(n) denote the statement

∴ P(1) is true.
Now assume that the statement be true for n = k
(i.e.,) assume P(k) be true

∴ P(k + 1) is true.
Thus if P(k) is true, P(k + 1) is true.
By the principle of Mathematical ‘induction, P(n) is true for all n ∈ N.

Question 3.
4 + 8 + 12 + ……. + 4n = 2n(n + 1), for all n ∈ N.
Solution:
Let P(n) denote the statement 4 + 8 + …….. + 4n = 2n(n + 1)
i.e., P(n) : 4 + 8 + 12 + … + 4n = 2n(n + 1)
Put n = 1,
P(1): LHS = 4
RHS = 2 (1)(1 + 1) = 4
P(1) is true.
Assume that P(n) is true for n = k
P(k): 4 + 8 + 12 + ……. + 4k = 2k(k + 1)
To prove P(k + 1)
i.e., to prove 4 + 8 + 12 + ……. + 4k + 4(k + 1) = 2(k + 1) (k + 1 + 1)
4 + 8 + 12 + …… + 4k + (4k + 4) = 2(k + 1) (k + 2)
Consider, 4 + 8 + 12 + …….. + 4k + (4k + 4) = 2k(k + 1) + (4k + 4)
= 2k(k + 1) + 4(k + 1)
= 2k² + 2k + 4k + 4
= 2k² + 6k + 4
= 2(k + 1)(k + 2)
P(k + 1) is also true.
∴ By Mathematical Induction, P(n) for all value n ∈ N.

Question 4.

Solution:

∴ P(1) is true.
Assume P(k) is true for n = k

∴ P(k + 1) is true whenever P(k) is true.
∴ By the Principle of Mathematical Induction, P(n) is true for all n ∈ N.

Question 5
3²ⁿ – 1 is divisible by 8, for all n ∈ N.
Solution:
Let P(n) denote the statement 3²ⁿ – 1 is divisible by 8 for all n ∈ N
Put n = 1
P(1) is the statement 3²⁽¹⁾ – 1 = 3² – 1 = 9 – 1 = 8, which is divisible by 8
∴ P(1) is true.
Assume that P(k) is true for n = k.
i.e., 3^2k – 1 is divisible by 8 to be true.
Let 3^2k – 1 = 8m
To prove P(k + 1) is true.
i.e., to prove 3^2(k+1) – 1 is divisible by 8
Consider 3^2(k+1) – 1 = 3^2k+2 – 1
= 3^2k . 3² – 1
= 3^2k (9) – 1
= 3^2k (8 + 1) – 1
= 3^2k × 8 + 3^2k × 1 – 1
= 3^2k (8) + 3^2k – 1
= 3^2k (8) + 8m (∵ 3^2k – 1 = 8m)
= 8(3^2k + m), which is divisible by 8.
∴ P(k + 1) is true wherever P(k) is true.
∴ By principle of Mathematical Induction, P(n) is true for all n ∈ N.

Question 6.
aⁿ – bⁿ is divisible by a – b, for all n ∈ N.
Solution:
Let P(n) denote the statement aⁿ – bⁿ is divisible by a – b.
Put n = 1. Then P(1) is the statement: a¹ – b¹ = a – b is divisible by a – b
∴ P(1) is true. Now assume that the statement be true for n = k
(i.e.,) assume P(k) be true, (i.e.,) a^k – b^k is divisible by (a – b) be true.

⇒ a^k – b^k = m(a – b)
⇒ a^k = b^k + m(a – b) ……. (1)
Now to prove P(k + 1) is true, (i.e.,) to prove: a^k+1 – b^k+1 is divisible by a – b
Consider a^k+1 – b^k+1 = a^k . a – b^k . b
= [b^k + m(a – b)] a – b^k . b [∵ a^k = bm + k(a – b)]
= b^k . a + am(a – b) – b^k . b
= b^k . a – b^k . b + am(a – b)
= b^k(a – b) + am(a – b)
= (a – b) (b^k + am) is divisible by (a – b)
∴ P(k + 1) is true.
By the principle of Mathematical induction. P(n) is true for all n ∈ N.
∴ aⁿ – bⁿ is divisible by a – b for n ∈ N.

Question 7.
5²ⁿ – 1 is divisible by 24, for all n ∈ N.
Solution:
Let P(n) be the proposition that 5²ⁿ – 1 is divisible by 24.
For n = 1, P(1) is: 5² – 1 = 25 – 1 = 24, 24 is divisible by 24.
Assume that P(k) is true.
i.e., 5^2k – 1 is divisible by 24
Let 5^2k – 1 = 24m
To prove P(k + 1) is true.
i.e., to prove 5^2(k+1) – 1 is divisible by 24.
P(k): 5^2k – 1 is divisible by 24.
P(k + 1) = 5^2(k+1) – 1
= 5^2k . 5² – 1
= 5^2k (25) – 1
= 5^2k (24 + 1) – 1
= 24 . 5^2k + 5^2k – 1
= 24 . 5^2k + 24m
= 24 [5^2k + 24]
which is divisible by 24 ⇒ P(k + 1) is also true.
Hence by mathematical induction, P(n) is true for all values n ∈ N.

Question 8.
n(n + 1) (n + 2) is divisible by 6, for all n ∈ N.
Solution:
P(n): n(n + 1) (n + 2) is divisible by 6.
P(1): 1 (2) (3) = 6 is divisible by 6
∴ P(1) is true.
Let us assume that P(k) is true for n = k
That is, k (k + 1) (k + 2) = 6m for some m
To prove P(k + 1) is true i.e. to prove (k + 1) (k + 2)(k + 3) is divisible by 6.
P(k + 1) = (k + 1) (k + 2) (k + 3)
= (k + 1)(k + 2)k + 3(k + 1)(k + 2)
= 6m + 3(k + 1)(k + 2)
In the second term either k + 1 or k + 2 will be even, whatever be the value of k.
Hence second term is also divisible by 6.
∴ P (k + 1) is also true whenever P(k) is true.
By Mathematical Induction P (n) is true for all values of n.

Question 9.
2ⁿ > n, for all n ∈ N.
Solution:
Let P(n) denote the statement 2ⁿ > n for all n ∈ N
i.e., P(n): 2ⁿ > n for n ≥ 1
Put n = 1, P(1): 2¹ > 1 which is true.
Assume that P(k) is true for n = k
i.e., 2^k > k for k ≥ 1
To prove P(k + 1) is true.
i.e., to prove 2^k+1 > k + 1 for k ≥ 1
Since 2^k > k
Multiply both sides by 2
2 . 2^k > 2k
2^k+1 > k + k
i.e., 2^k+1 > k + 1 (∵ k ≥ 1)
∴ P(k + 1) is true whenever P(k) is true.
∴ By principal of mathematical induction P(n) is true for all n ∈ N.