Let the 3 consecutive co-efficients be
(nr−1),(nr),(nr+1)And these are in the ratio 1:7:35 so(nr−1)=17⋅(nr)(1)and(nr+1)=5⋅(nr)(2)
(nr−1)=17⋅(nr)(1) LHS=n!(r−1)!(n−r+1)! LHS=n!r!(r)(n−r)!(n−r+1) LHS=n!(r)r!(n−r)!(n−r+1) LHS=(nr)(r)(n−r+1) so17=rn−r+1n−r+1=7rn=8r−1
Now simplify equation 2 and then solve simulataneously to find n and r. Then check your answer
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LaTex:
\binom{n}{r-1},\qquad \binom{n}{r},\qquad \binom{n}{r+1}\\
\text{And these are in the ratio }\qquad 1:7:35\\~\\
so\\
\binom{n}{r-1}=\frac{1}{7}\cdot \binom{n}{r}\qquad \color{red}{(1)} \color{black}{\quad and } \qquad
\binom{n}{r+1}=5\cdot \binom{n}{r}\quad \color{red}{(2)}\\~\\
\binom{n}{r-1}=\frac{1}{7}\cdot \binom{n}{r}\qquad \color{red}{(1)} \\~\\
LHS=\frac{n!}{(r-1)!(n-r+1)!}\\~\\
LHS=\frac{n!}{\frac{r!}{(r)}(n-r)!(n-r+1)}\\~\\
LHS=\frac{n!\qquad (r)}{r!(n-r)!\quad (n-r+1)}\\~\\
LHS=\binom{n}{r}\frac{ (r)}{ (n-r+1)}\\~\\
so\\
\frac{1}{7}=\frac{r}{n-r+1}\\
n-r+1=7r\\
n=8r-1