a. $VT = a(b-c)-b(a+c) = ab -ac - ab - bc = -ac-bc = (-c)(b+c)=VP^{}$
b. $VT = a(b+c)-b(a-c) = ab + ac - ab + bc = ac + bc = c(a+b)=VP^{}$
c. $VT = a(b-c)-a(b+d) = ab -ac - ab - ad = -ac - ad = (-a)(c+d)=VP^{}$
d. $VT= (a+b)(c+d)-(a+d)(b+c) = ac + ad + bc + bd - ( ab + ac + bd +cd) = ac + ad + bc + bd - ab -ac - bd - cd = ad + bc - ab - cd = (ad - ab) + (bc - cd) = a(d-b) - c(-b+d) = ( a-c)(d-b)=VP^{}$