Sum Difference Identities
1. **State the problem:** Given the equations $x + y = 8$ and $x^2 + y^2 = 34$, we need to find:
- $x^2 + 2xy + y^2$
- $2xy$
- $x^2 - 2xy + y^2$
- The values of $x$ and $y$
2. From the first equation, square both sides:
$$ (x + y)^2 = 8^2 $$
$$ x^2 + 2xy + y^2 = 64 $$
This gives us the value for $x^2 + 2xy + y^2$ as 64.
3. We know $x^2 + y^2 = 34$ from the second equation.
Using the identity from step 2:
$$ x^2 + 2xy + y^2 = 64 $$
$$ x^2 + y^2 + 2xy = 64 $$
Substitute $x^2 + y^2 = 34$:
$$ 34 + 2xy = 64 $$
Solve for $2xy$:
$$ 2xy = 64 - 34 = 30 $$
4. Find $x^2 - 2xy + y^2$:
Using the identity:
$$ x^2 - 2xy + y^2 = (x - y)^2 $$
Rewrite:
$$ x^2 + y^2 - 2xy $$
Substitute known values:
$$ 34 - 30 = 4 $$
So, $x^2 - 2xy + y^2 = 4$.
5. Find the values of $x$ and $y$:
We have:
$$ (x + y)^2 = 64 $$
$$ (x - y)^2 = 4 $$
Take square roots:
$$ x + y = 8 $$
$$ |x - y| = 2 $$
There are two cases:
**Case 1:**
$$ x - y = 2 $$
Add this to $x + y = 8$:
$$ 2x = 10 \\ x = 5 $$
Find $y$:
$$ 5 + y = 8 \\ y = 3 $$
**Case 2:**
$$ x - y = -2 $$
Add to $x + y = 8$:
$$ 2x = 6 \\ x = 3 $$
Find $y$:
$$ 3 + y = 8 \\ y = 5 $$
So, the values of $(x,y)$ are $(5,3)$ or $(3,5)$.
**Final answers:**
- $x^2 + 2xy + y^2 = 64$
- $2xy = 30$
- $x^2 - 2xy + y^2 = 4$
- $x=5, y=3$ or $x=3, y=5$