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Basically, could somebody please explain to me how I find [tex]{\varphi}_u_u[/tex], I really don't understand how it's come about. Apparantly I need to use the chain rule again and the product rule but I don't understand how to, if somebody could show me explicitly how to use the product rule in this case I'd be very greatful. The bit under the line is the solution but it's not obvious how they get to that. Thanks for any help!

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# Higher order partial derivatives and the chain rule

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