I'm new to Mathematica and I keep getting this error(in the title) when I'm trying to solve this definite integral numerically. This is for a math class where we have to use the arc length.
f[x_] := 6 x[1 - x]
NIntegrate[Sqrt[1. + f'[x]^2], {x, 0., 1.}]
Any help would be appreciated. I've googled several times and while a lot of people are getting the same error, their solutions/problems aren't really the same as mine. My instructor showed us this exact syntax I am pretty sure and he was getting the correct answer.
The error is
NIntegrate::inumr: The integrand Sqrt[1. +36 (1[1+Times[<<2>>]]-(x^[Prime])[Plus[<<2>>]])^2] has evaluated to non-numerical values for all sampling points in the region with boundaries {{0.,1.}}.
EDIT: Thank you. I thought it was brackets for some reason.
f[x_] := 6 x(1 - x)
: Parentheses for grouping, square brackets for function calls. $\endgroup$