/--это дробь
1)6s/12y3c*6y2c=6s/12y3*6y2=s/2y3*6y2=s/2y*6=s/y*3=3s/y
2)25с3d/4c6:20cd/c6=25c3d/4c6*c6/20cd=25c3/4c6*c6/20c=25c2/4*1/20=5c2/4*1/4=5c2/16
3) 25c+20/c-2*c2-4/25c2-16=5(5c+4)/c-2*(c-2)*(c+2)/(5c-4)*(5c+4)=5/c-2*(c-2)*(c+2)/5c-4=5*c+2/5c-4
4) r+s/s*(s/r-s/r+s)=r+s/s*s*(r+s)-rs/r*(r+s)=r+s/s*rs+s2-rs/r*(r+s)=1/s*rs+s2-rs/r=1/s*s2/r=s/r