Page:The Algebra of Mohammed Ben Musa (1831).djvu/93

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If you know only one of the diagonals, then you are aware, that there are two triangles, two sides of each of which have every one five yards, while the third is the diagonal. Hereafter you can make the computation according to the rules for the triangles.^[1] This is the figure:

The fourth kind, or Rhomboid, is computed in the same way as the rhombus. Here is the figure to it:

The other quadrangles are calculated by drawing a (57) diagonal, and computing them as triangles.

Triangles are of three kinds, acute-angular, obtuse-angular, or rectangular. The peculiarity. of the rectangular triangle is, that if you multiply each of its two short sides by itself, and then add them together, their sum will be equal to the long side multiplied by itself. The character of the acute-angled triangle is

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1. If the two diagonals are $d$ and $d'$, and the side $s$, the area of the rhombus is ${\tfrac {dd'}{2}}=d\times {\sqrt {s^{2}-{\tfrac {d^{2}}{4}}}}$.