Indices - Surds and Logarithms
3. SURDS
3.3. Rationalisation of a surd
If the product of two surds is a rational number, then they are said to be rationalization factor to each other, for example \(9\sqrt 7 \) and \(5\sqrt 7 \) are rationalized to each other as \(9\sqrt 7 \times 5\sqrt 7 = 315\), a rational number. Similarly \(\sqrt 5 + \sqrt 3 \) and \(\sqrt 5 - \sqrt 3 \) are rationalizing factor of each other.
Again \(\left( {{5^{1/3}} + {3^{1/3}}} \right)\) and \(\left( {{5^{2/3}} + {3^{2/3}} - {{15}^{1/3}}} \right)\) are rationalizing factor of each other.
Example 01: Find the simplified value of \(\frac{1}{{\sqrt {14} + \sqrt 9 }} + \frac{1}{{\sqrt {19} + \sqrt {14} }} + .... + \frac{1}{{\sqrt {324} + \sqrt {319} }}\)
Example 02: If \(\frac{1}{{\sqrt[3]{7} + \sqrt[3]{3}}} = a\sqrt[3]{{49}} + b\sqrt[3]{9} + c\sqrt[3]{{21}}\), find the value of \(a,\;b\) and \(c\)
Hence \(\cfrac{1}{{{7^{1/3}} + {3^{1/3}}}} = \cfrac{{({7^{2/3}} + {3^{2/3}} - {7^{1/3}}{3^{1/3}})}}{{(7 + 3)}}\) \( = \cfrac{1}{{10}}\sqrt[3]{{49}} + \cfrac{1}{{10}}\sqrt[3]{9} - \cfrac{1}{{10}}\sqrt[3]{{21}}\).
Thus \(a = b = \frac{1}{{10}},\,c = - \cfrac{1}{{10}}\).