1)
(12 / (x - 1)) / (6 / x)
= 6(2 / (x - 1)) / 6(1 / x)
= (2 / (x - 1)) / (1 / x)
= x(2 / (x - 1)) / x(1 / x)
= (2x / (x - 1)) / 1
= 2x / (x - 1)
10)
[(1/y) + (1/(y - 1))] / [(1/y) - (2/(y - 1))]
= (y(y - 1))[(1/y) + (1/(y - 1))] / (y(y - 1))[(1/y) - (2/(y - 1))]
= [(y(y - 1))(1/y) + (y(y - 1))(1/(y - 1))] / [(y(y - 1))(1/y) - (y(y - 1))(2/(y - 1))]
= [y - 1 + y] / [y - 1 - 2y]
= [2y - 1] / [-y - 1]
= -[1 - 2y] / -[y + 1]
= (1 - 2y) / (1 + y)