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