Which condition is necessary for a triangle ABCABCABC to have an angle of 90∘90^\circ90∘?
a2+b2=c2a^2+b^2=c^2a2+b2=c2
sinA+sinB=sinC\sin A + \sin B = \sin CsinA+sinB=sinC
cosA+cosB=cosC\cos A + \cos B = \cos CcosA+cosB=cosC
a+b=ca+b=ca+b=c