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If x3 - y3 = 112 and x - y = 4, then what is the value of x2 + y2?
Options:
16
20
24
28
Let’s break this down step by step:
- You’re told: x³ - y³ = 112 and x - y = 4.
- Remember this formula: x³ - y³ = (x - y)(x² + xy + y²). So, plugging in, 112 = 4(x² + xy + y²).
- That gives us: x² + xy + y² = 28.
- Also, x² + y² = (x - y)² + 2xy ? x² + y² = 16 + 2xy.
- Now, from earlier: x² + xy + y² = 28.
- But x² + y² = ? So, rearrange: (x² + y²) + xy = 28, meaning x² + y² = 28 - xy.
- Substitute x² + y² from the previous line into this to get:
16 + 2xy = 28 - xy ? 3xy = 12 ? xy = 4.
- Substitute xy = 4 back in: x² + y² = 16 + 2×4 = 24.
Now, look at the options:
- 1: 16 2: 20 3: 24 4: 28
So, the correct answer is Option 3: 24
Here’s what matters:
- We used the cubic factoring identity.
- The rest is just plugging values into basic algebra.
- 24 is the number that checks out; the other options are just there to distract you.
You got it right!
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