Find the first partial derivatives of the function. (Sn = x1 + 2x2 + ... + nxn; i = 1, ..., n. Give your answer only in terms of Sn and i.) u = sin(x1 + 2x2 + ⋯ + nxn)

Jack has a piece of red ribbon that is two times as long as his piece of blue ribbon. he says that he can use two different equation to find out how long his piece of red ribbon is compared to his piece of blue ribbon is. is he correct? explain his reasoning