Correct Answer - Option 2 : -144√5
Given:
a = \(\frac{{2\; - \;\sqrt 5 }}{{2\; + \;\sqrt 5 }}\)
b = \(\frac{{2\; + \;\sqrt 5 }}{{2\; - \;\sqrt 5 }}\)
Formula Used:
a2 – b2 = (a + b) (a – b)
(a + b)2 = a2 + b2 + 2ab
(a – b)2 = a2 + b2 – 2ab
Calculation:
a + b = \(\frac{{2\; - \;\sqrt 5 }}{{2\; + \;\sqrt 5 }}\) + \(\frac{{2\; + \;\sqrt 5 }}{{2\; - \;\sqrt 5 }}\)
⇒ a + b = \(\frac{{{{\left( {2\; - \;\sqrt 5 } \right)}^2}\; + \;{{\left( {2\; + \;\sqrt 5 } \right)}^2}}}{{\left( {2\; + \;\sqrt 5 } \right)\left( {2\; - \;\sqrt 5 } \right)}}\)
⇒ a + b = \(\frac{{4\; + \;5\; - \;4\sqrt 5 \; + \;4\; + \;5\; + \;\;4\sqrt 5 }}{{4\; - \;5}}\)
⇒ a + b = 18/(–1)
⇒ a + b = (–18)
a – b = \(\frac{{2\; - \;\sqrt 5 }}{{2\; + \;\sqrt 5 }}\) – \(\frac{{2\; + \;\sqrt 5 }}{{2\; - \;\sqrt 5 }}\)
⇒ a – b = \(\frac{{{{\left( {2\; - \;\sqrt 5 } \right)}^2}\; - \;{{\left( {2\; + \;\sqrt 5 } \right)}^2}}}{{\left( {2\; + \;\sqrt 5 } \right)\left( {2\; - \;\sqrt 5 } \right)}}\)
⇒ a – b = \(\frac{{4\; + \;5\; - \;4\sqrt 5 \; - \;\left( {4\; + \;5\; + \;4\sqrt 5 \;} \right)}}{{4\; - \;5}}\)
⇒ a – b = (–8√ 5 )/(–1)
⇒ a – b = 8√ 5
a2 – b2 = (–18) × (8√5)
⇒ a2 – b2 = (–144√5)
∴ The value of a2 – b2 is –144√5