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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).