Which of the following represents a valid identity for all real a,ba, ba,b?
a2+b2=a+b\sqrt{a^2 + b^2} = a + ba2+b2=a+b
(a+b)2=a2+b2(a + b)^2 = a^2 + b^2(a+b)2=a2+b2
(a+b)(a−b)=a2−b2(a + b)(a - b) = a^2 - b^2(a+b)(a−b)=a2−b2
ab+ba=1\frac{a}{b} + \frac{b}{a} = 1ba+ab=1 for ab≠0ab \neq 0ab=0