S.T. the time rate of the total angular momentum of a system of particles about a point is equal to the sum of the external torques acting on the system taken about the same point.

Explanation

For a system of n particles,
Net angular momentum `=vecL=vecL_1+vecL_2+……..vecL_n=sumvecr_ixxvecp_i`
Now `(dvecL)/(dt)=sumd/(dt)vecr_ixxvecp_i=sumvecr_ixxvecF_i "where" (dvecp)/(dt)=vecF=sumtau_i "but" vectau_("int")=0 and sum vectau_i=vectau_("int")+vectau_("txt")`
`therefore (dvecL)/(dt)=vectau_("ext")`.