Solo debes reemplazar el valor en cada letra
a) 4ab - 5b + 3ab ..........................., si a =3,b =2/3,c = -1
4.3.2/3 - 5.2/3 + 3.3.2/3 =
8 - 10/3 + 6 =
14 - 10/3 =
(42 - 10)/3 = 32/3
b) ( x+y )( x-y ) + 2xy ,............................. si x = 1/4 ,y = -2/5
( 1/4+ -2/5 )( 1/4 -(- 2/5) ) + 2(1/4)(-2/5)
-3/20 * 13/20 + (-4/20) =
10/20 - 4/20 = 6/20 = 3/10
espero que te sirva, salu2!!!