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Question
If $$x = a + \frac{1}{a}$$   and $$y = a - \frac{1}{a},$$   then the value of x4 + y4 - 2x2y2 is?
Options:
A .  4
B .  8
C .  16
D .  64
Answer: Option C
$$\eqalign{
& x = a + \frac{1}{a} \cr
& {x^2} = {a^2} + \frac{1}{{{a^2}}} + 2 \cr
& {y^2} = {a^2} + \frac{1}{{{a^2}}} - 2 \cr
& {\text{Now, }} \cr
& {x^4} + {y^4} - 2{x^2}{y^2} \cr
& = {\left( {{x^2} - {y^2}} \right)^2} \cr
& = {\left( {{a^2} + \frac{1}{{{a^2}}} + 2 - {a^2} - \frac{1}{{{a^2}}} + 2} \right)^2} \cr
& = {\left( 4 \right)^2} \cr
& = 16 \cr} $$

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