#y=5/x^7#
#y=a/b" "y^'=(a^'*b-b^'*a)/b^2#
#(d y)/(d x)=(0*x^7-7*x^6*5)/x^(14)#
#(d y)/(d x)=-(35x^6)/x^14#
#(d y)/(d x)=-(35)/x^8" "=-35*x^-8#
#" or :"#
#y=5*x^-7#
#y=a*b#
#y^'=a^'*b+b^'*a#
#(d y)/(d x)=-7*0*x^-8-7*x^-8*5#
#(d y)/(d x)=0-35x^-8#
#(d y)/(d x)=-35x^-8#