Appendix 1: Proofs in Mathematics | A1.3

Question 1

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

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Solution

Let's use algebra to represent any two consecutive odd numbers.

Step 1 — Representing the numbers

We need to pick any odd number. Let's use a variable for this. An odd number can be written as 2n+12n + 1. Here, nn is any whole number. The next consecutive odd number is 22 more than the first. So, we add 2 to our first number.

(2n+1)+2(2n + 1) + 2

=2n+3= 2n + 3

Second odd number=2n+3\boxed{\text{Second odd number} = 2n + 3}

Diagram 1

Step 2 — Finding their sum

Now, let's add these two consecutive odd numbers. We will sum the first odd number and the second odd number.

(2n+1)+(2n+3)(2n + 1) + (2n + 3)

=2n+1+2n+3= 2n + 1 + 2n + 3

=4n+4= 4n + 4

We can see that 4 is a common factor here. Let's factor out 4 from the sum.

=4(n+1)= 4(n + 1)

Since nn is a whole number, n+1n+1 is also a whole number. This means the sum is 4 multiplied by a whole number. Any number that can be written as 4 times a whole number is divisible by 4.

Answer

The sum of two consecutive odd numbers is divisible by 4.

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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