To simplify the expression given in the math problem, let's proceed step by step.

The expression in question is:

\[
(x^3 + x^2 y + y^3)(x + y) - 2x^2 y^2 - 3x^2 y^2 - 2xy^3
\]

a) **Rút gọn biểu thức:**

1. Begin with expanding \((x^3 + x^2y + y^3)(x + y)\):
\[
= x^3 \cdot x + x^3 \cdot y + x^2y \cdot x + x^2y \cdot y + y^3 \cdot x + y^3 \cdot y
\]
\[
= x^4 + x^3y + x^2y^2 + xy^2 + xy^3 + y^4
\]

2. Combine similar terms:
\[
= x^4 + x^3y + (x^2y^2 - 2x^2y^2 - 3x^2y^2) + y^4 - 2xy^3
\]

3. Combine like terms:
\[
= x^4 + x^3y + y^4 - 6x^2y^2 - 2xy^3
\]

Thus, the simplified expression is:

\[
x^4 + x^3y + y^4 - 6x^2y^2 - 2xy^3
\]

b) **Tính giá trị của biểu thức với \(x = -1\) và \(y = -1\)**:

Substituting \(x = -1\) and \(y = -1\) into the simplified expression:
\[
(-1)^4 + (-1)^3(-1) + (-1)^4 - 6(-1)^2(-1)^2 - 2(-1)(-1)^3
\]
Calculating each term:
\[
= 1 + 1 + 1 - 6(1)(1) - 2(1)
\]
\[
= 1 + 1 + 1 - 6 - 2
\]
\[
= 3 - 8 = -5
\]

The value of the expression at \(x = -1\) and \(y = -1\) is \(-5\).