If $$x = \root 3 \of {a + \sqrt {{a^2} + {b^3}} } $$ + $$\root 3 \of {a - \sqrt {{a^2} + {b^3}} } $$ then $${x^3} + 3bx$$ is equal to?
A. 0
B. a
C. 2a
D. 1
Answer: Option C
Solution(By Examveda Team)
$${\text{Given}}\,x = \root 3 \of {a + \sqrt {{a^2} + {b^3}} } \,\,+ $$ $$\root 3 \of {a - \sqrt {{a^2} + {b^3}} } $$$$\eqalign{ & {\text{Let}}\,z = \sqrt {{a^2} + {b^3}} \cr & \therefore x = \root 3 \of {a + z} + \root 3 \of {a - z} \cr & {\text{Cubing both side}} \cr} $$
$$\therefore {x^3} = {\left( {\root 3 \of {a + z} } \right)^3} + {\left( {\root 3 \of {a - z} } \right)^3} \, + $$ $$\,3{\left( {a + z} \right)^{\frac{1}{3}}}$$ $${\left( {a - z} \right)^{\frac{1}{3}}} \times\, $$ $$\left( {\root 3 \of {a + z} + \root 3 \of {a - z} } \right)$$
$$ \,\Rightarrow {x^3} = a + z + a - z \,+\, $$ $$3{\left( {{a^2} + az - az - {z^2}} \right)^{\frac{1}{3}}}$$ $$ \times \left( x \right)$$
$$\eqalign{ & \Rightarrow {x^3} = 2a + 3{\left( {{a^2} - {z^2}} \right)^{\frac{1}{3}}} \times \left( x \right) \cr & {\text{Put the value of }}z \cr & \Rightarrow {x^3} = 2a + 3{\left( {{a^2} - {a^2} - {b^3}} \right)^{\frac{1}{3}}} \times \left( x \right) \cr & \Rightarrow {x^3} = 2a + 3{\left( { - {b^3}} \right)^{\frac{1}{3}}} \times \left( x \right) \cr & \Rightarrow {x^3} = 2a - 3bx \cr & \therefore {x^3} + 3bx = 2a \cr} $$
Related Questions on Algebra
If $$p \times q = p + q + \frac{p}{q}{\text{,}}$$ then the value of 8 × 2 is?
A. 6
B. 10
C. 14
D. 16
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D. $$\frac{{x + 4}}{x}$$
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C. $$\frac{6}{8}$$
D. $$\frac{8}{6}$$
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