The factored forms and the result of the division are as follows:
- \(5x^4 + 20x^3 - 105x^2 = 5x^2(x - 3)(x + 7)\)
- \(x^4 + 8x^2 + 15 = (x^2 + 3)(x^2 + 5)\)
- \(\frac{5x^3 - 4x - 1}{x + 1} = 5x^2 - 5x + 1\) with a remainder of \(-2\)
Thus, the final boxed answers are:
\[
\boxed{5x^4 + 20x^3 - 105x^2 = 5x^2(x - 3)(x + 7)}
\]
\[
\boxed{x^4 + 8x^2 + 15 = (x^2 + 3)(x^2 + 5)}
\]
\[
\boxed{\frac{5x^3 - 4x - 1}{x + 1} = 5x^2 - 5x + 1 \text{ with remainder } -2}
\]