a
(a) Given determinant
$ = \left| {\,\begin{array}{*{20}{c}}1&x&{x + 1}\\{2x}&{x(x - 1)}&{(x + 1)\,x}\\{3x(x - 1)}&{x(x - 1)\,(x - 2)}&{(x + 1)\,x(x - 1)}\end{array}\,} \right|$
$ = x(x + 1)\,\left| {\,\begin{array}{*{20}{c}}1&x&1\\{2x}&{x - 1}&{\,x}\\{3x(x - 1)}&{(x - 1)\,(x - 2)}&{x(x - 1)}\end{array}\,} \right|$
= $x(x + 1)\,(x - 1)\,\left| {\,\begin{array}{*{20}{c}}1&1&1\\{2x}&{x - 1}&x\\{3x}&{x - 2}&x\end{array}\,} \right|$
Applying ${C_1} - {C_3}$ and ${C_2} - {C_3}$
$x(x + 1)\,(x - 1)\,\left| {\,\begin{array}{*{20}{c}}0&0&1\\x&{ - 1}&x\\{2x}&{ - 2}&x\end{array}\,} \right|$=$x(x + 1)(x - 1)\,[ - 2x + 2x] = 0$
$\therefore $ $f(x) = 0 \Rightarrow f(100) = 0$.