3) (c^2-d^2)/2c^2 * (cd+d^2)/(c+d)^2 =
=(c-d)(c+d)d(c+d)/2c^2(c+d)(c+d)=
=(cd-d^2)/2c^2
5) a+b/a-b * a^2-ab/2a^2-2b^2 =
=(a+b)a(a-b)/(a-b)2(a^2-b^2)=
=a(a^2-b^2) / 2(a-b)(a^2-b^2)=
= a/2(a-b)
7)xy+y^2/a^2-b^2:x^2-y^2/a+b=
y(x+y)(a+b)/(a-b)(a+b)(x-y)(x+y)=
y/(a-b)(x-y)