Math Problem Statement

If a + 1/a = 3 1/3 find the value of a ^ 3 - 1/(a ^ 3)

Solution

We are given the equation:

a+1a=313=103a + \frac{1}{a} = 3 \frac{1}{3} = \frac{10}{3}

We need to find the value of:

a31a3a^3 - \frac{1}{a^3}

Step 1: Let a+1a=xa + \frac{1}{a} = x

Given that x=103x = \frac{10}{3}, we now need to express a31a3a^3 - \frac{1}{a^3} in terms of xx.

Step 2: Use the identity for cubes

The identity that relates a31a3a^3 - \frac{1}{a^3} to a+1aa + \frac{1}{a} is:

a31a3=(a+1a)(a21+1a2)a^3 - \frac{1}{a^3} = \left( a + \frac{1}{a} \right) \left( a^2 - 1 + \frac{1}{a^2} \right)

First, we need to find the value of a2+1a2a^2 + \frac{1}{a^2}. We can use the square of a+1aa + \frac{1}{a}:

(a+1a)2=a2+2+1a2\left( a + \frac{1}{a} \right)^2 = a^2 + 2 + \frac{1}{a^2}

So,

(103)2=a2+2+1a2\left( \frac{10}{3} \right)^2 = a^2 + 2 + \frac{1}{a^2}

1009=a2+2+1a2\frac{100}{9} = a^2 + 2 + \frac{1}{a^2}

Subtract 2 from both sides:

a2+1a2=10092=1009189=829a^2 + \frac{1}{a^2} = \frac{100}{9} - 2 = \frac{100}{9} - \frac{18}{9} = \frac{82}{9}

Step 3: Find a31a3a^3 - \frac{1}{a^3}

Now, use the identity:

a31a3=(a+1a)(a21+1a2)a^3 - \frac{1}{a^3} = \left( a + \frac{1}{a} \right) \left( a^2 - 1 + \frac{1}{a^2} \right)

Simplifying this, we find:

a31a3=(103)(829)a^3 - \frac{1}{a^3} = \left( \frac{10}{3} \right) \left( \frac{82}{9} \right)

Multiply these fractions:

a31a3=10×823×9=82027a^3 - \frac{1}{a^3} = \frac{10 \times 82}{3 \times 9} = \frac{820}{27}

Thus, the value of a31a3a^3 - \frac{1}{a^3} is:

82027\boxed{\frac{820}{27}}

Would you like more details, or do you have any questions?

Here are some related questions to explore:

  1. How does this formula change if a+1aa + \frac{1}{a} equals a different value?
  2. Can you derive similar identities for higher powers of aa?
  3. What if a+1aa + \frac{1}{a} were a complex number?
  4. How can you use the same approach for negative exponents?
  5. Can you derive a general formula for an1ana^n - \frac{1}{a^n}?

Tip: When solving equations with fractions, always consider simplifying expressions using identities for sums, products, and powers.

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Math Problem Analysis

Mathematical Concepts

Algebra
Equations
Cube Roots

Formulas

Identity for cubes: a^3 - 1/a^3 = (a + 1/a)(a^2 - 1 + 1/a^2)

Theorems

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Suitable Grade Level

Advanced High School