1.

For all a, b ∈ N, we define a * b = a3 + b3. Show that * is commutative but not associative.

Answer»

let a = 1,b = 2∈N 

a*b = 13 + 23 = 9 

And b*a = 23 + 13 = 9 

Hence * is commutative. 

Let c = 3 

(a*b)*c = 9*c = 9 3 + 3

a*(b*c) = a*(23 + 33) = 1*35 = 13 + 35

(a*b)*c ≠ a*(b*c) 

Hence * is not associative.



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