x^2+x-30=0
x^2-5x+6x-30=0
x(x-5)+6(x-5)=0
(x-5)(x+6)=0
=>x-5=0 và x+6=0
x=5 ,x=-6
b)(x+y+z)^3-x^3-y^3-z^3
=(x+y+z)^3-(x^3+y^3+z^3)
=(x+y+z)^3-[(x+y)^3-3xy(x+y)+z^3]
=(x+y+z)^3-[(x+y+z)^3-3(x+y)z(x+y+z)-3xy(x+y)]
=(x+y+z)^3-(x+y+z)^3+3(x+y)z(x+y+z)+3xy(x+y)
=3(x+y)(xz+yz+z^2+xy)