1.a^(2*3)*a^(2*4)/a^(2*5)=a^6*a^8/a^10=a^6+8-10=a^4
2.{(x^2+y^2+2xy)-(x^2+y^2-2xy)}/2xy
={x^2+y^2+2xy-x^2-y^2+2xy}/2xy
=4xy/2xy
=2