(x^2+ax+1)(x^2+bx+1)
= x^2.x^2 + x^2.bx + x^2 + ax.x^2 + ax.bx + ax + x^2 + bx + 1
= x^4 + bx^3 + x^2 + ax^3 + abx^2 + ax + x^2 + bx + 1
= x^4 + bx^3 + 2x^2 + ax^3 + abx^2 + ax + bx + 1
= x^4 + (bx^3 + ax^3) + (2x^2 + abx^2) + (ax + bx) + 1
= x^4 + x^3.(a + b) + x^2.(2 + ab) + x.(a + b) + 1
= x^4 + [x^3.(a + b) + x.(a + b)] + x^2.(2 + ab) + 1
= x^4 + x.(a + b).(x^2 + 1) + x^2.(2 + ab) + 1
#chucbanhoctotnhe;333
