let a, b, c, d be Element of COMPLEX ; :: thesis: angle a,b,c = angle (a + d),(b + d),(c + d)
thus angle a,b,c = angle (a - b),0c ,(c - b) by Th85
.= angle ((a + d) - (b + d)),0c ,((c + d) - (b + d))
.= angle (a + d),(b + d),(c + d) by Th85 ; :: thesis: verum