Math Problem Statement

if 3x + 2y = 5 and xy = - 2 , find 27x³ + 8y³

Solution

We are given two equations:

  1. 3x+2y=53x + 2y = 5
  2. xy=2xy = -2

We need to find the value of 27x3+8y327x^3 + 8y^3.

Step-by-Step Solution:

Step 1: Recognize the form of the expression.

The expression 27x3+8y327x^3 + 8y^3 can be rewritten as:

27x3+8y3=(3x)3+(2y)327x^3 + 8y^3 = (3x)^3 + (2y)^3

Step 2: Use the identity for sum of cubes.

We use the identity for the sum of cubes:

a3+b3=(a+b)(a2ab+b2)a^3 + b^3 = (a + b)(a^2 - ab + b^2)

Here, let a=3xa = 3x and b=2yb = 2y. So we have:

(3x)3+(2y)3=(3x+2y)((3x)2(3x)(2y)+(2y)2)(3x)^3 + (2y)^3 = (3x + 2y)((3x)^2 - (3x)(2y) + (2y)^2)

Step 3: Substitute 3x+2y3x + 2y and simplify.

From the first equation, we know that 3x+2y=53x + 2y = 5. Substituting this, we get:

(3x)3+(2y)3=5((3x)2(3x)(2y)+(2y)2)(3x)^3 + (2y)^3 = 5 \left( (3x)^2 - (3x)(2y) + (2y)^2 \right)

Now, we calculate each term inside the parentheses:

  • ( (3x)^2 =

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Math Problem Analysis

Mathematical Concepts

Algebra
Cubic Expressions
Sum of Cubes

Formulas

Sum of cubes identity: a^3 + b^3 = (a + b)(a^2 - ab + b^2)

Theorems

Sum of Cubes Theorem

Suitable Grade Level

Grades 9-11