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NDA 2026
Exam
NDA
Year
2026
Subject
Mathematics
Chapter
Binomial Theorem

NDA 2026 · Mathematics · Binomial Theorem

Passage: Let u be a positive integer and f be a real number lying...

Passage: Let uu be a positive integer and ff be a real number lying between 0and10 and 1. Further, \left(\sqrt{2}+1\right)^{10}=u+fand(2−1)10=vand \left(\sqrt{2}-1\right)^{10}=v.

Question: What is the value of (v+f)(v+f) ?

Options

  1. A22
  2. B11
  3. C0⋅50\cdot5
  4. D0⋅250\cdot25

Correct answer

B. 11

Step-by-step solution

Given (2+1)10=u+f(\sqrt{2}+1)^{10} = u+f and (2−1)10=v(\sqrt{2}-1)^{10} = v.

Adding both equations:

u+f+v=(2+1)10+(2−1)10u+f+v = (\sqrt{2}+1)^{10} + (\sqrt{2}-1)^{10}

Expanding using the binomial theorem:

u+f+v=2[10C0(2)10+10C2(2)8+⋯+10C10]u+f+v = 2 [^{10}C_0 (\sqrt{2})^{10} + ^{10}C_2 (\sqrt{2})^8 + \dots + ^{10}C_{10}]

The right hand side is an even integer. Let it be 2k2k.

u+f+v=2ku+f+v = 2k

f+v=2k−uf+v = 2k - u

Since 2k2k and uu are integers, f+vf+v must be an integer.

We are given 0<f<10 < f < 1.

Also, 0<2−1<1⇒0<(2−1)10<1⇒0<v<10 < \sqrt{2}-1 < 1 \Rightarrow 0 < (\sqrt{2}-1)^{10} < 1 \Rightarrow 0 < v < 1.

Adding the inequalities for ff and vv:

0<f+v<20 < f+v < 2

Since f+vf+v is an integer strictly between 00 and 22, the only possible value is 11.

Therefore, v+f=1v+f = 1.

Answer: 11
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