# If a1, a2, a3 and a4 are the coefficient of any four consecutive terms in the expansion of (1 + x)n, prove that a1a1+a2+a3a3+a4=2a2a2+a3 - Mathematics

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If a1, a2, a3 and a4 are the coefficient of any four consecutive terms in the expansion of (1 + x)n, prove that (a_1)/(a_1 + a_2) + (a_3)/(a_3 + a_4) = (2a_2)/(a_2 + a_3)

#### Solution

Let a1, a2, a3 and a4 be the coefficient of four consecutive terms "T"_(r + 1), "T"_(r + 2), "T"_(r + 3) and "T"_(r + 4) respectively.

Then a1 = coefficient of Tr+1 = nCr

a2 = coefficient of Tr+2 = nCr+1

a3 = coefficient of Tr+3 = nCr+2

And a4 = coefficient of Tr+4 = nCr+3

Thus (a_1)/(a_1 + a_2) = (""^n"C"_r)/(""^n"C"_r + ""^n"C"_(r + 1))

= (""^n"C"_r)/(""^(n + 1)"C"_(r + 1)   .....(because ""^n"C"_r + ""^n"C"_(r + 1) = ""^(n + 1)"C"_(r + 1))

= (n)/(r(n - r)) xx  ((r + 1)(n - r))/(n + 1)

= (r + 1)/(n + 1)

Similarly, (a_3)/(a_3 + a_4) = (""^n"C"_(r + 2))/(""^n"C"_(r + 2) + ""^n"C"_(r + 3))

= (""^n"C"_(r + 2))/(""^(n + 1)"C"_(r + 3))

= (r + 3)/(n + 1)

Hence, L.H.S. = a_1/(a_1 + a_2) + a_3/(a_3 + a_4)

= (r + 1)/(n + 1) + (r + 3)/(n + 1)

= (2r + 4)/(n + 1)

And R.H.S. = (2a_2)/(a_2 + a_2) + a_3/(a_3 + a_4)

= (2(""^n"C"_(r + 1)))/(""^n"C"_(r + 1) + ""^n"C"_(r + 2))

= (2(""^n"C"_(r + 1)))/(""^(n + 1)"C"_(r + 2))

= 2  n/((r + 1)(n - r - 1)) xx ((r + 2)(n - r - 1))/(n + 1)

= (2(r + 2))/(n + 1)

Concept: Binomial Theorem for Positive Integral Indices
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#### APPEARS IN

NCERT Mathematics Exemplar Class 11
Chapter 8 Binomial Theorem
Solved Examples | Q 15 | Page 138
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