a) -(-a+c-d)-(c-a+d)=a-c+d-c+a-d=2a-2c
b) -(a+b-c+d)+(a-b-c-d)=-a-b+c-d+a-b-c-d=-2b-2d
c) a(b-c-d)-a(b+c-d)=ab-ac-ad-ab-ac+ad=-2ac
d) (a+b)(c+d)-(a+d)(b+c)=ac+ad+bc+bd-ab-ac-bd-cd=ad+bc-ab-bd
e) (a+b)(c-d)-(a-b)(c+d)=ac-ad+bc-bd-ac-ad+bc+bd=-2ad+2bc
f) 2(a+b)-2(a-b)=2a+2b-2a+2b=4b