If x + 1/{4x} =3/2 , find the value of 8x^3 + 1/ {8x^3 }

RRB NTPC Mains or Stage 2 Model Paper 8 with solutions

Question : 39 of 120

Marks: +1, -0

x + 1/{4x} =3/2

, find the value of

8x^3 + 1/ {8x^3 }

Solution:

(a)

x +1 /{4x }=3/2⇒2x +1/{2x}= 3

Cubing both sides,

8x^3 +1/8x^ 3+ 3 × 2x ×1/{2x}

[2x+ 1/{2x}]= 27

⇒ 8x^3 +1/{8x^ 3}+3 × 3 = 27

⇒ 8x^3 + 1/{8x^3}+= 27 – 9 = 18

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