Chapter 1: Problem 8
Show that $$ \frac{1}{a+b \sqrt{c}}=\frac{a-b \sqrt{c}}{a^{2}-b^{2} c} $$ Hence express the following numbers in the form \(x+y \sqrt{n}\) where \(x\) and \(y\) are rational numbers and \(n\) is an integer: (a) \(\frac{1}{7+5 \sqrt{2}}\) (b) \(\frac{2+3 \sqrt{2}}{9-7 \sqrt{2}}\) (c) \(\frac{4-2 \sqrt{3}}{7-3 \sqrt{3}}\) (d) \(\frac{2+4 \sqrt{5}}{4-\sqrt{5}}\)
Short Answer
Step by step solution
Rationalizing the Denominator
Expressing \( \frac{1}{7+5\sqrt{2}} \) in the Form \( x+y\sqrt{n} \)
Expressing \( \frac{2+3\sqrt{2}}{9-7\sqrt{2}} \) in the Form \( x+y\sqrt{n} \)
Expressing \( \frac{4-2\sqrt{3}}{7-3\sqrt{3}} \) in the Form \( x+y\sqrt{n} \)
Expressing \( \frac{2+4\sqrt{5}}{4-\sqrt{5}} \) in the Form \( x+y\sqrt{n} \)
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Conjugate Pairs
The key idea is to multiply the numerator and denominator of a fraction by the conjugate of the denominator. For instance, if the denominator is \( a + b\sqrt{c} \), multiply by its conjugate \( a - b\sqrt{c} \). This multiplication utilizes the difference of squares identity:
- \((a + b\sqrt{c})(a - b\sqrt{c}) = a^2 - (b\sqrt{c})^2\)
Simplifying Radical Expressions
When simplifying an expression like \( \frac{1}{a + b\sqrt{c}} \), the radical and any potential surds need converting into more controlled terms. By using conjugate pairs for rationalization, one can convert the expression into its simplest compatible form.
- Combine like terms where possible.
- Convert surds or complex numbers into rational numbers using the conjugate method.
Algebraic Manipulation
In the process of rationalizing denominators or simplifying expressions, algebraic manipulation is key. For example:
- Expanding products: \((a+b)(c+d) = ac + ad + bc + bd\).
- Factoring to simplify expressions: \(x^2 + 5x + 6 = (x+2)(x+3)\).