The seven real roots of the equation x7+28x6+308x5+1680x4+4704x3+6272x2+3136x−1282+256=0 are, in ascending order:
x1=−a−bcos(dcπ)x2=−a−bcos(deπ)x3=−a−bcos(dfπ)x4=−a−bcos(dgπ)x5=−a+bcos(dhπ)x6=−a+jjx7=−a+bcos(dkπ)
All the variables are positive integer numbers, and the angles are in radians between 0 and 2π. Find a+b+c+d+e+f+g+h+j+k.