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Section 2.8 in Till Tantau's manual for TikZ tells me that I can manage styles - predefined sets of options that can be used to organize how a graphic is drawn.

I am trying to set my grid to be in red by manipulating help lines :

\documentclass[tikz,border=2mm]{standalone}
\begin{document}
\begin ...

@rais - Thank you. It works.

I want my filldraw circle to be placed at coordinates (x,y)=(cos(30°), sin(30°)) - a point on the unit circle with angle 30°.
These are (actually) equivalent to (x,y)=(√3/2,1/2), or approximately (0.866,0.5).

So, how can I assign these coordinate as (cos(30°), sin(30°)) to my node, label, or any ...