let a, b, c, d be 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 Th69
.= angle (((a + d) - (b + d)),0c,((c + d) - (b + d)))
.= angle ((a + d),(b + d),(c + d)) by Th69 ; :: thesis: verum