1) (a-2)/a-1 - a/1-a = (a-2+a)/a-1 = (2a-2)/a-1= 2(a-1)/a-1 =2
2)(3y+7)/4-y + (y+15)/y-4 = (3y+7-y-15)/4-y=(2y-8)/4-y = 2(y-4)/4-y = -2
3)(2a-3)^2/9a-27 + (a-6)^2/27-9a = (2a-3)^2-(a-6)^2/9a-27= (4a^2-12a+9-a^2+12a-36)/9a-27= (3a^2-27)/9a-27= (a^2-9)/3a-9=(a-3)(a+3)/3(a-3)=(a+3)/3
4)(25-3x)/(x-5)^2 - (7x-x^2)/(5-x)^2=(25-3x-7x+x^2)/(x-5)^2 =(x^2-10x+25)/(x-5)^2 =(x-5)^2 /(x-5)^2 = 1