no 12,14 sama 15 yah kawan.tolong bantu ya kawan

12)
(x^-1 - y^-1)/(x^-1 + y^-1)
= (1/x - 1/y)/(1/x + 1/y)
= (y/xy - x/xy)/(y/xy + x/xy)
= ((y-x)/xy)/((y+x)/xy)
= ((y-x)/xy) × (xy/(y+x)).....(xy dicoret)
= (y-x)/(y+x) (d)

14)
(ab^-1 - a^-1b)/(a^-1 + b^-1)
= (a/b - b/a)/(1/a + 1/b)
= ((a^2)/ab - (b^2)/ab)/(b/ab + a/ab)
= ((a^2 - b^2)/ab)/((a + b)/ab)
= ((a^2 - b^2)/ab) × (ab/(a + b))..(ab dicoret)
= (a^2 - b^2)/(a + b)
= ((a + b)(a - b))/(a + b)...((a+b) dicoret)
= a - b (b)