a^3+b^3 =(a+b)(a^2-ab+b^2)=(a+b)[(a-b)^2+ab]= orgreater than (a+b)ab
b^3+c^3 is=or greater than (b+c)bc
c^3+a^3 is =or greaterthan(c+a)ca
by addition we get the required statement