Ex 1.1 , 5
Use Euclid’s division lemma to show that the cube of any positive integer is of the form 9m, 9m+ 1 or 9m+ 8.
As per Euclid’s Division Lemma
If a and b are 2 positive integers, then
a = bq + r
where 0 ≤ r < b
Let positive integer be a
And b = 3
Hence a = 3q + r
where ( 0 ≤ r < 3)
r is an integer greater than or equal to 0 and less than 3
hence r can be either 0 , 1 or 2
If r = 0
Our equation becomes
a = 3q + r
a = 3q + 0
a = 3q
Cubing both sides
a3 = (3q)3
a3 = 27q3
a3 = 9(3q3)
a3 = 9m
Where m = 3q3
If r = 1
Our equation becomes
a = 3q + r
a = 3q + 1
Cubing both sides
a3 = (3q + 1)3
a3 = (3q)3 + 13 +
3 × 3q × 1(3q + 1)
a3 = 27q3 + 1 + 9q × (3q + 1)
a3 = 27q3 + 1 + 27q2 + 9q
a3 = 27q3 + 27q2 + 9q + 1
a3 = 9(3q3 + 3q2 + q) + 1
a3 = 9m + 1
Where m = 3q3 + 3q2 + q
If r = 2
Our equation
becomes
a = 3q + r
a = 3q + 2
Cubing both sides
a3 = (3q + 2)3
a3 = (3q)3 + 23 +
3 × 3q × 2(3q + 2)
a3 = 27q3 + 8 + 18q × (3q + 2)
a3 = 27q3 + 8 + 18q2 + 6q
a3 = 27q3 + 54q2 + 36q + 8
a3 = 9(3q3 + 6q2 + 4q) + 8
a3 = 9m + 8
Where m = 3q3 + 6q2 + 4q
Hence, cube of any positive number can be expressed of the
form 9m or 9m + 1 or 9m + 8
Hence proved
Made by
Davneet Singh
Davneet Singh has done his B.Tech from Indian Institute of Technology, Kanpur. He has been teaching from the past 14 years. He provides courses for Maths, Science and Computer Science at Teachoo
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