MCQ
$(\sqrt{5}+1)^4+(\sqrt{5}-1)^4$ is
  • A
    an irrational number
  • B
    a negative real number
  • a rational number
  • D
    a negative integer

Answer

Correct option: C.
a rational number
We have $(a+b)^n+(a-b)^n$
$=\left[{ }^n C_0 a^n+{ }^n C_1 a^{n-1} b+{ }^n C_2 a^{n-2}
b^2+{ }^n C_3 a^{n-3} b^3+\ldots . .+{ }^n C_n b^n\right]+$
${\left[{ }^n
C_0 a^n-{ }^n C_1 a^{n-1} b+{ }^n C_2 a^{n-2} b^2-{ }^n C_3 a^{n-3}
b^3+\ldots . .+(-1)^n \cdot{ }^n C_n b^n\right]}$
$=2\left[{ }^n C_0 \quad
a^n+{ }^n C_2 a^{n-2} b^2+\ldots\right]$
Let $a =\sqrt{5}$ and $b =1$ and $n =4$
Now we get $(\sqrt{5}+1)^4+(\sqrt{5}-1)^4$
$=2\left[{ }^4 C_0(\sqrt{5})^4+{ }^4C_2(\sqrt{5})^2 1^2+{ }^4 C_4(\sqrt{5})^0 1^4\right]$
$=2[25+30+1]=112$

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