103.
The sum of the co-efficients of all odd degree terms in the expansion of $${\left( {x + \sqrt {{x^3} - 1} } \right)^5} + {\left( {x - \sqrt {{x^3} - 1} } \right)^5},\left( {x > 1} \right)$$ is:
$${T_{r + 1}} = {\,^{256}}{C_r}{\left( {\sqrt 3 } \right)^{256 - r}}{\left( {\root 8 \of 5 } \right)^r} = {\,^{256}}{C_r}{\left( 3 \right)^{\frac{{256 - r}}{2}}}{\left( 5 \right)^{\frac{r}{8}}}$$
Terms will be integral if $$\frac{{256 - r}}{2}\,\& \,\frac{r}{8}$$ both are $$+ve$$ integer, which is so if $$r$$ is an integral multiple of 8. As $$0 \leqslant r \leqslant 256$$
$$\therefore r = 0,8,16,24,.....256,{\text{ total }}33{\text{ values}}{\text{.}}$$
105.
If $$P_n$$ denotes the product of the binomial coefficients in the expansion of $${\left( {1 + x} \right)^n},\,$$ then $$\frac{{{P_{n + 1}}}}{{{P_n}}}$$ equals
$$\eqalign{
& {T_n} = \frac{{1 \cdot 3 \cdot 5.....\left( {2n - 1} \right)}}{{\left( {2n} \right)!}} \cr
& = \frac{{1 \cdot 2 \cdot 3 \cdot 4.....\left( {2n - 1} \right) \cdot 2n}}{{\left( {2n} \right)!\,2 \cdot 4 \cdot 6.....2n}} \cr
& = \frac{{\left( {2n} \right)!}}{{\left( {2n} \right)!\,{2^n}n!}} = \frac{1}{{{2^n}n!}} \cr} $$
Now putting $$n = 1, 2, 3, .....$$ we see that the sum of series
$$\eqalign{
& S = \frac{1}{2} + \frac{{{{\left( {\frac{1}{2}} \right)}^2}}}{{2!}} + \frac{{{{\left( {\frac{1}{2}} \right)}^3}}}{{3!}} + ..... \cr
& = {e^{\frac{1}{2}}} - 1 = \sqrt e - 1 \cr} $$
107.
If $$'n'$$ is positive integer and three consecutive coefficient in the expansion of $${\left( {1 + x} \right)^n}$$ are in the ratio $$6 : 33 : 110$$ then n is equal to :
108.
The co-efficient of the middle term in the binomial expansion in powers of $$x$$ of $${\left( {1 + \alpha x} \right)^4}\,{\text{and of }}{\left( {1 - \alpha x} \right)^6}$$ is the same if $$\alpha $$ equals
$${t_{r + 1}} = {\,^{100}}{C_r}{\left( {\root 8 \of 5 } \right)^{100 - r}} \cdot {\left( {\root 6 \of 2 } \right)^r}.$$
As 2 and 5 are coprime, $${t_{r + 1}}$$ will be rational if $$100 - r$$ is a multiple of 8 and $$r$$ is a multiple of 6. Also $$0 \leqslant r \leqslant 100.$$
$$\eqalign{
& \therefore \,\,r = 0,6,12,.....,96 \cr
& \therefore \,\,100 - r = 4,10,16,.....,100\,\,\,\,\,\,.....\left( 1 \right) \cr} $$
But $$100 - r$$ is to be a multiple of 8.
So, $$100 - r = 0,8,16,24,.....,96\,\,\,\,\,\,\,\,.....\left( 2 \right)$$
The common terms in (1) and (2) are 16, 40, 64 and 88.
∴ $$r = 84, 60, 36, 12$$ give rational terms.
∴ the number of irrational terms = $$101 - 4 = 97.$$