Which of these equations is true ?

b = a
b*a = a*a
b*a - b*b = a*a - b*b
b (a - b) = (a + b)*(a - b)
b = a + b
b = 2*b
1 = 2


x = 0,999...
10x = 9,999...
10x - x = 9,999... - 0,999...
9x = 9
x = 1
0,999... = 1