Coefficient of $x^3$ in $(1-2x+3x^2-4x^3)^{1/2}$ The question is to find out the coefficient of $x^3$ in the expansion of  $(1-2x+3x^2-4x^3)^{1/2}$
I tried using multinomial theorem but here the exponent is a fraction and I couldn't get how to proceed.Any ideas?

Hint:
The coefficient of $x^3$ in the expansion of  $(1-2x+3x^2-4x^3)^{1/2}$
$=$
the coefficient of $x^3$ in the expansion of  $(1-2x+3x^2-4x^3+\cdots)^{1/2}$
Using Calculating $1+\frac13+\frac{1\cdot3}{3\cdot6}+\frac{1\cdot3\cdot5}{3\cdot6\cdot9}+\frac{1\cdot3\cdot5\cdot7}{3\cdot6\cdot9\cdot12}+\dots? $,
$$1-2x+3x^2-4x^3+\cdots=(1+x)^{-2}$$
$$(1-2x+3x^2-4x^3+\cdots)^{1/2}=(1+x)^{-1}=1-x+x^2-x^3+\cdots$$
