JAMB - Mathematics (2008 - No. 20)
A binary operation * is defined on the set of positive integers is such x*y = 2x-3y+2 for all positive integers x and y. The binary operation is?
commutative and close on the set of positive integers
neither commutative nor closed on the set of positive integers
commutative but not closed on the set of positive integers
not cummutative but closed on the set of positive integers
解释
X * Y = 2X - 3Y + 2
2*3 = 2(2) - 3(3) + 2
=4-9+2
= -3
But -3 does not belong to positive integer
2*3 = 2(2) - 3(3) + 2
=4-9+2
= -3
But -3 does not belong to positive integer