= First moment of area
{title2=$M_x=\iint_Ry\,dA,\quad M_y=\iint_Rx\,dA$}
For a planar region $R$ of finite positive <area> $A$, its first moments of area about the coordinate axes are the displayed <double integrals>, provided they exist. Its <centroid> is $(\overline x,\overline y)=(M_y/A,M_x/A)$. This follows by applying the definition $\overline{\mathbf r}=A^{-1}\iint_R\mathbf r\,dA$ to each component. The subscript indicates the axis about which the moment is taken: the first moment about the $x$-axis weights by $y$, not $x$.
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