#f_((x))=6x^(1/2)(x^2+x)^2-8x^(3/2)-8x^(1/2)=#
#=6(x^4+2x^3+x^2)*x^(1/2)-8x*x^(1/2)-8*x^(1/2)=#
#=x^(1/2)[6(x^4+2x^3+x^2)-8x-8]=#
#=[6x^4+12x^3+6x^2-8x-8]sqrtx=#
#=2(sqrtx)(3x^4+6x^3+3x^2-4x-4)=#
#=2(sqrtx)[3x^2(x^2+2x+1)-4(x+1)]=#
#=2(sqrtx)[3x^2(x+1)^2-4(x+1)]=#
#=2(sqrtx){(x+1)[3x^2(x+1)-4]}=#
#=2(x+1)(sqrtx)[3x^2(x+1)-4]#