$a)777775.777780.777773-777777.777772.777779$
$=777775.(777775+5).(777775-2)-(777775+2).(777775-3).(777775+4)$
$\text{Đặt 777775 = x, thì ta có:}$
$⇒777775.(777775+5).(777775-2)-(777775+2).(777775-3).(777775+4)$
$=x(x+5)(x-2)-(x+2)(x-3)(x+4)$
$=x(x^2+3x-10)-(x^2-x-6)(x+4)$
$=x^3+3x^2-10x-(x^3+4x^2-x^2-4x-6x-24)$
$=x^3+3x^2-10x-x^3-4x^2+x^2+4x+6x+24$
$=24.$
$b)888887.888892.888885-888889.888884.888891$
$=(888890-3).(888890+2).(888890-5)-(888890-1).(888890-6).(888890+1)$
$\text{Đặt 888890 = y, thì ta có:}$
$⇒(888890-3).(888890+2).(888890-5)-(888890-1).(888890-6).(888890+1)$
$=(y-3)(y+2).(y-5)-(y-1)(y-6)(y+1)$
$=(y^2+2y-3y-6)(y-5)-(y^2-1)(y-6)$
$=(y^2-y-6)(y-5)-(y^2-1)(y-6)$
$=y^3-5y^2-y^2+5y-6y+30-(y^3-6y^2-y+6)$
$=y^3-5y^2-y^2+5y-6y-30-y^3+6y^2+y-6$
$=24.$
