10
(x-4)(x+4)/3(x+4)=(x+4)/3
13
(a-2)/2-(a-3)/3-1=(3a-6-2a+6-6)/6=(a-6)/6
15
((x+3)²/2(x-2)*(x-2)(x+2)/3(x+3)=(x+3)(x+2)/6
16
(x-25)/5(x-5)+(3x+5)/x(x-5)=(x²-25x+15x+25)/5x(x-5)=
=(x²-10x+25)/5x(x-5)=(x-5)²/5x(x-5)=(x-5)/5x
18
a²/(a+1)-a+1=(a²-a²-a+a+1)/(a+1)=1/(a+1)