10.19.3 Can you answer these questions?

10.19.3.1 Sum doesn't work as I expected

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=303934

I'm trying to sum a bunch of polynomials. I'm implementing that: 
 Sum[Subscript[P, k][c] (k + x) z^(k + x), {k, -x, -1}] 
 
and mathematica gives me back that 
 
I ... 
[bugs] [summation]
 

asked by Felipe https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=11690 Score of 2

10.19.3.2 How to find the local maximum and minimum of a function

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=304011

I am trying to find the local maximum and minimum of a function. The code below works if function is an exponential but when function involves a sinusoid, it does not give numerical values and instead ... 
[calculus-and-analysis]
 

asked by kahless96 https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=99169 Score of 1

answered by Bob Hanlon https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=9362 Score of 0

10.19.3.3 Error message: Coefficient : 0 is not a valid variable in DSolve

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=303940

I have a simple differential equation 
 test1[x_] = 
DSolveValue[{f''[x] + Log[x]*f[x] == 0, f[0] == 1, f'[0] == -1}, 
f[x], x] 
 
which for some reason gives me an ... 
[differential-equations] [warning-messages] [singularity]
 

asked by Confuse-ray30 https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=96205 Score of 1