Ta có: \\(f\\left(x\\right)=x^4+8x^3+14x^2-8x-15\\)
= \\(x^4+4x^3+4x^3+3x^2+16x^2-5x^2+12x-20x-15\\)
= \\(\\left(x^4+4x^3+3x^2\\right)+\\left(4x^3+16x^2+12x\\right)-\\left(5x^2+20x+15\\right)\\)
= \\(x^2\\left(x^2+4x+3\\right)+4x\\left(x^2+4x+3\\right)-5\\left(x^2+4x+3\\right)\\)
= \\(\\left(x^2+4x+3\\right)\\left(x^2+4x-5\\right)\\)
\\(=(x+1)(x+3)(x-1)(x+5)\\)