Tough Gradient Proof. Help!

I was asked execute the proof shown in figure 1. I managed to prove what is in figure 2, but cannot make any headway on figure 3. Please Help.

Hello,

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Originally Posted by Jensen

I was asked execute the proof shown in figure 1. I managed to prove what is in figure 2, but cannot make any headway on figure 3. Please Help.

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Remember that
And hence

Thanks for the help. I also found a vector identity that worked nicely also.