a^2+b^2=13
(a+b)=5
(a+b)^2=a^2+b^2+2ab
ab=[ (a+b)^2-(a^2+b^2) ] / 2 = [ 5^2-13 ] / 2 = 6
(a-b)^2=(a+b)^2-4ab=5^2-4*6=1
a-b=±1