Question
Question: The number of points in the Cartesian plane with integral co-ordinates satisfying the inequalities \...
The number of points in the Cartesian plane with integral co-ordinates satisfying the inequalities |x| £ k, |y| £ k, |x – y| £ k ; is-
A
(k + 1)3 – k3
B
(k + 2)3 – (k +1)3
C
(k2 + 1)
D
None of these
Answer
(k + 1)3 – k3
Explanation
Solution
|x| £ k Ž –k £ x £ k ….(1)
& |y| £ k Ž –k £ y £ k ….(2)
& |x – y| £ k Ž |y – x| £ k
Ž –k £ y – x £ k Ž x – k £ y £ x + k ….(3)
\ Number of points having integral coordinates
= (2k + 1)2 – 2 [k + (k – 1) + …. + 2 + 1]
= (3k2 + 3k + 1).