a, tai x = 5 va y =2
x^2y +5xy^2 = 5^2 . 2 + 5 . 5 . 2^2 = 150
b=99^99-(99+1)98+(99+1)9997-(99+1).9996+..........+(99+1)99-1
=9999-9999-9998+9998+9997-9997-9996+.................+992+99-1
=99-1
=98
minh bổ sung thêm cho vu thi thu 2002 là y=-2,vậy A=25.-2+5.5.4=50