How to evaluate a derivative of a function at a point?

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Mike M
Mike M 2015-4-16
编辑： Mohammad Abouali 2015-4-16
I was wondering if anyone can help. I am taking the derivative of cos(x)-x, and then using subs to evaluate it at a point, but instead of getting an answer it returns me this:
syms x
func = @(x) cos(x) - x
newfunc = (subs(diff(func,x,1),x,1))
newfunc =
- sin(1) - 1
I found an answer online of putting vpa() before subs, which makes it work, but I was wondering why subs isn't able to evaluate it. Thanks!

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Mohammad Abouali
Mohammad Abouali 2015-4-16
编辑：Mohammad Abouali 2015-4-16
because subs replaces x with one. Doesn't evaluate. So diff( cos(x) -x ) returns -sin(x) -1 and then you replace x with one and returns -sin(1) -1
syms x
func = @(x) cos(x) - x
newfunc = eval( (subs(diff(func,x,1),x,1)) )
newfunc =
-1.8414709848079

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