let c1, c2 be Element of COMPLEX ; :: thesis: ( c2 <> 0 & (Arg c2) - (Arg c1) >= 0 implies angle c1,c2 = (Arg c2) - (Arg c1) )
assume A1: ( c2 <> 0 & (Arg c2) - (Arg c1) >= 0 ) ; :: thesis: angle c1,c2 = (Arg c2) - (Arg c1)
hence angle c1,c2 = Arg (Rotate c2,(- (Arg c1))) by COMPLEX2:def 5
.= (Arg c2) - (Arg c1) by A1, Lm10 ;
:: thesis: verum