By integration. This means the plane is divided into small pieces, and the contribution of each individual piece to the moment of inertia is evaluated. There are mathematical methods to do this more or less easily - systematically, at least - for certain simple figures, and you can find the moment of inertial of many common figures published in lists.
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find the strength of the member subject to bending or shear. Moment of inertia is used to find radius of gyratia or flexural regidity so that member strength flexural stress is found
length x width
you count the number of boxes IN the shape.
Yes, if it is bound by plane figures, just add the area of each plane figure. If it has a curved surface, divide it into many small pieces, to approximate the area with small rectangles or triangles, then add them up.
There is no such formula. To determine the area of an irregular plane shape split it into known shapes such as rectangles, triangles, segments of a circle etc. Determine the area(s) of each of these and sum the results to find the total area.