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I'm trying to write Tikz that will draw lines on a circle at regular intervals (in order to slice it) and have used this code:

\begin{tikzpicture} \draw (0,0) circle(2); \draw plot[domain=pi:2*pi] ({2*cos(\x r)},{.4*sin(\x r)}); \draw[dashed] plot[domain=0:pi] ({2*cos(\x r)},{.4*sin(\x r)}); \foreach \y in {-2, -1.8, ..., 2}{ \draw (sqrt{4-\y*\y},\y) -- (-1 * sqrt{4-\y*\y},\y); } \end{tikzpicture}

however this code produces: the incorrect result

which as you can guess, isn't the result I'm looking for, where did I go wrong?

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    write \draw ({sqrt(4-\y*\y)},\y) -- ({-1 * sqrt(4-\y*\y)},\y); instead of \draw (sqrt{4-\y*\y},\y) -- (-1 * sqrt{4-\y*\y},\y); Commented Feb 22 at 23:36
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    The sqrt uses parentheses for its arguments. The entire coordinate component must be enclosed in curly braces, since your logic used parentheses. Commented Feb 22 at 23:37

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As Jasper pointed out in their comment, your syntax for calculating the coordinates of the slice endpoints is wrong. Try it this way:

\begin{tikzpicture}[ declare function={f(\x) = sqrt(4-(\x)^2);} ] \draw (0,0) circle(2); \draw plot[domain=pi:2*pi] ({2*cos(\x r)},{.4*sin(\x r)}); \draw[dashed] plot[domain=0:pi] ({2*cos(\x r)},{.4*sin(\x r)}); \foreach \y in {-2, -1.8, ..., 2}{ \draw ({-f(\y)},\y) -- ({f(\y)},\y); } \end{tikzpicture}

I like the declare function key for this because you can write out the formula once and refer to it by name.

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