The given partial differential equation is:
\( \frac{\partial^2 z}{\partial x^2} – 2\frac{\partial^2 z}{\partial x \partial y} + \frac{\partial^2 z}{\partial y^2} = x + 2y \)
This is a second-order linear non-homogeneous PDE. The general solution \( z(x,y) \) is the sum of the complementary solution \( z_c(x,y) \) and a particular solution \( z_p(x,y) \).
**1. Complementary Solution \( z_c(x,y) \):**
First, consider the homogeneous equation:
\( \frac{\partial^2 z}{\partial x^2} – 2\frac{\partial^2 z}{\partial x \partial y} + \frac{\partial^2 z}{\partial y^2} = 0 \)
Using operator notation \( D_x = \frac{\partial}{\partial x} \) and \( D_y = \frac{\partial}{\partial y} \), the equation becomes:
\( (D_x^2 – 2D_x D_y + D_y^2) z = 0 \)
This can be factored as:
\( (D_x – D_y)^2 z = 0 \)
The auxiliary equation is obtained by replacing \( D_x \) with \( m \) and \( D_y \) with \( 1 \):
\( m^2 – 2m + 1 = 0 \)
\( (m-1)^2 = 0 \)
This gives a repeated root \( m = 1 \).
For a repeated root \( m \), the complementary solution is given by:
\( z_c(x,y) = f_1(y+mx) + x f_2(y+mx) \)
Substituting \( m=1 \):
\( z_c(x,y) = f_1(y+x) + x f_2(y+x) \)
where \( f_1 \) and \( f_2 \) are arbitrary differentiable functions.
**2. Particular Solution \( z_p(x,y) \):**
The non-homogeneous term is \( F(x,y) = x+2y \), which is a first-degree polynomial. The differential operator is \( (D_x – D_y)^2 \), which is of degree 2.
Since \( F(x,y) \) is a polynomial of degree 1 and the operator is of degree 2, we assume a particular solution \( z_p \) that is a polynomial of degree \( 1+2=3 \).
Let \( z_p = Ax^3 + Bx^2y + Cxy^2 + Dy^3 \).
Now, we compute the partial derivatives:
\( \frac{\partial z_p}{\partial x} = 3Ax^2 + 2Bxy + Cy^2 \)
\( \frac{\partial z_p}{\partial y} = Bx^2 + 2Cxy + 3Dy^2 \)
\( \frac{\partial^2 z_p}{\partial x^2} = 6Ax + 2By \)
\( \frac{\partial^2 z_p}{\partial y^2} = 2Cx + 6Dy \)
\( \frac{\partial^2 z_p}{\partial x \partial y} = 2Bx + 2Cy \)
Substitute these into the original PDE:
\( (6Ax + 2By) – 2(2Bx + 2Cy) + (2Cx + 6Dy) = x + 2y \)
\( 6Ax + 2By – 4Bx – 4Cy + 2Cx + 6Dy = x + 2y \)
Group the terms by \( x \) and \( y \):
\( (6A – 4B + 2C)x + (2B – 4C + 6D)y = x + 2y \)
Comparing the coefficients of \( x \) and \( y \) on both sides:
1. Coefficient of \( x \): \( 6A – 4B + 2C = 1 \)
2. Coefficient of \( y \): \( 2B – 4C + 6D = 2 \)
We have 2 equations and 4 unknowns. We can choose values for two of the unknowns to simplify. Let’s choose \( C=0 \) and \( D=0 \).
From equation (1): \( 6A – 4B = 1 \)
From equation (2): \( 2B = 2 \implies B = 1 \)
Substitute \( B=1 \) into \( 6A – 4B = 1 \):
\( 6A – 4(1) = 1 \)
\( 6A – 4 = 1 \)
\( 6A = 5 \)
\( A = \frac{5}{6} \)
So, a particular solution is \( z_p(x,y) = \frac{5}{6}x^3 + x^2y \).
**3. General Solution:**
The general solution is the sum of the complementary and particular solutions:
\( z(x,y) = z_c(x,y) + z_p(x,y) \)
\( z(x,y) = f_1(y+x) + x f_2(y+x) + \frac{5}{6}x^3 + x^2y \)
The final answer is $\boxed{z(x,y) = f_1(y+x) + x f_2(y+x) + \frac{5}{6}x^3 + x^2y}$.