I am having trouble making the jump from my understanding to a given solution. Given the equation:
$E_D = \frac12 (t - \sum_{i=1}^n \delta_iw_iI_i)^2$
I factor this out to:
$\frac{\partial E_D}{\partial w_i} = \frac12 (t - \sum_{i=1}^n \delta_iw_iI_i)^2 = -t\delta_iI_i+\delta_iI_i\sum_{i=1}^n\delta_iw_iI_i$
But the answer I am given is:
$\frac{\partial E_D}{\partial w_i} = \frac12 (t - \sum_{i=1}^n \delta_iw_iI_i)^2 = -t\delta_iI_i+ w_i\delta_i^2I_i^2 + \sum_{j=1,j\ne i}^n\delta_i\delta_jI_iI_jw_i$
I am not sure what I am missing, can anyone help?