SSC CGL 16 Aug 2015 Shift 2 Paper

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Question : 134 of 200

Marks: +1, -0

If 4a -

4/a

+ 3 = 0 then the value of :

a^3-1/a^3

+ 3 = ?

$3/{16}$
$7/{16}$
${21}/{64}$
${21}/{16}$

Solution:

4a -

4/a

= - 3
On dividing by 4,
⇒ a -

1/a

= -

3/4

∴

a^3 -1/a^3

=

(a-1/a)^3

+ 3a ×

1/a(a-1/a)

=

(-3/4)^2

+ 3 ×

-3/4

= -

{27}/{64}

-

9/4

=

{-27-144}/{64}

=

{-171}/{64}

∴

a^3 - 1/a^3

+ 3 =

{-171}/{64}

+ 3
=

{-171+192}/{64}

=

{21}/{64}

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