Appendix 1: Proofs in Mathematics | A1.3

Question 2

Take two consecutive odd numbers. Find the sum of their squares, and then add 6 to the result. Prove that the new number is always divisible by 8.

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Solution

We will represent the consecutive odd numbers algebraically and simplify the expression.

Step 1 — Representing the numbers

Let's pick an integer nn. We can write any odd number as 2n12n - 1. The next consecutive odd number is 2n+12n + 1. We will use these two expressions.

Diagram 1

Step 2 — Sum of their squares

First, we square the first odd number. (2n1)2(2n - 1)^2 =(2n)22(2n)(1)+12= (2n)^2 - 2(2n)(1) + 1^2 =4n24n+1= 4n^2 - 4n + 1

Next, we square the second odd number. (2n+1)2(2n + 1)^2 =(2n)2+2(2n)(1)+12= (2n)^2 + 2(2n)(1) + 1^2 =4n2+4n+1= 4n^2 + 4n + 1

Now, we add these two squared results together. (4n24n+1)+(4n2+4n+1)(4n^2 - 4n + 1) + (4n^2 + 4n + 1) =4n2+4n24n+4n+1+1= 4n^2 + 4n^2 - 4n + 4n + 1 + 1 =8n2+2= 8n^2 + 2

8n2+2\boxed{8n^2 + 2}

Step 3 — Add 6 to the result

We take the sum of squares and add 6 to it. (8n2+2)+6(8n^2 + 2) + 6 =8n2+8= 8n^2 + 8

8n2+8\boxed{8n^2 + 8}

Step 4 — Proving divisibility by 8

Let's look at the new number we found. We can factor out a common term. 8n2+88n^2 + 8 =8(n2+1)= 8(n^2 + 1)

Since nn is an integer, n2n^2 is an integer. So, n2+1n^2 + 1 is also an integer. This means the entire expression is 8 multiplied by an integer. Any number that can be written as 8 times an integer is divisible by 8.

Answer

(i) The algebraic representation of two consecutive odd numbers is 2n12n-1 and 2n+12n+1. (ii) The sum of their squares plus 6 is 8(n2+1)8(n^2 + 1). (iii) Since the expression is 88 multiplied by an integer, it is always divisible by 88.

More questions in A1.3

Q1

Prove that the sum of two consecutive odd numbers is divisible by 4.

Q2

Take two consecutive odd numbers. Find the sum of their squares, and then add 6 to the result. Prove that the new number is always divisible by 8.

Q3

If p5p \ge 5 is a prime number, show that p2+2p^2 + 2 is divisible by 3.

[Hint: Use Example 11].

Q4

Let xx and yy be rational numbers. Show that xyxy is a rational number.

Q5

If aa and bb are positive integers, then you know that a=bq+r,0r<ba = bq + r, 0 \le r < b, where qq is a whole number. Prove that HCF(a,b)=HCF(b,r)\text{HCF}(a, b) = \text{HCF}(b, r).

[Hint : Let HCF(b,r)=h\text{HCF}(b, r) = h. So, b=k1hb = k_1h and r=k2hr = k_2h, where k1k_1 and k2k_2 are coprime.]

Q6

A line parallel to side BC of a triangle ABC, intersects AB and AC at D and E respectively.

Prove that ADDB=AEEC\frac{\text{AD}}{\text{DB}} = \frac{\text{AE}}{\text{EC}}.

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