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basic_tools:pythagorean_theorem [2018/03/27 16:10] jakobadmin created |
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<tabbox Intuitive> | <tabbox Intuitive> | ||
- | <note tip> | + | The Pythagorean theorem states a relationship between the three sides of a right-angled triangle where $a$ and $b$ represent the sides that form the right angle and $c$ is the longest side. |
- | Explanations in this section should contain no formulas, but instead colloquial things like you would hear them during a coffee break or at a cocktail party. | + | |
- | </note> | + | |
| | ||
<tabbox Concrete> | <tabbox Concrete> | ||
- | + | $a^2+b^2=c^2$ **if and only if** the triangle is a right triangle | |
- | <note tip> | + | |
- | In this section things should be explained by analogy and with pictures and, if necessary, some formulas. | + | |
- | </note> | + | |
<tabbox Abstract> | <tabbox Abstract> | ||
- | <note tip> | + | <blockquote> |
- | The motto in this section is: //the higher the level of abstraction, the better//. | + | |
- | </note> | + | Pythagoras’s Theorem, |
+ | $$(ds)^2 = (dx)^2 + (dy)^2 $$ | ||
+ | |||
+ | Riemann realized some fundamental things: | ||
+ | |||
+ | - Pythagoras’s Theorem not only told of the properties of right-angled triangles, but the function $(ds)$ (the positive square root of $(ds)^2$), was the ‘distance’ between two points; | ||
+ | - (ii) more than that, it was the shortest ‘distance’ between two points; | ||
+ | - (iii) most amazing of all, Pythagoras’s $(ds)^2$ was but one possibility - more generally, it could be written as: | ||
+ | $$(ds)^2 = g_{xx}(dx)(dx) + +g_{xy}(dx)(dy) + g_{yx}(dy)(dx) + g_{yy}(dy)(dy) ,$$ | ||
+ | where the $g_{ij}$ are coefficients to be determined. In Pythagoras’s Theorem, it just so happens that we have the values $g_{xx}=1$, $g_{xy}=0$, $g_{yx}=0$, and $g_{yy}=1$. | ||
+ | |||
+ | In other cases (other spaces),the $g_{ij}$ need not have these particular values, and need not be constants - they could even be functions of x and y. This discussion has, so far, been in a space of only two dimensions, but Riemann generalized his ‘distance function’ still further to n dimensions. [...] | ||
+ | Most remarkable of all, Riemann showed that all the geometric properties of an n-dimensional space can be completely determined by just this ‘distance’, $ds$ (the square root of the above equation. For example, if any of the coefficients, $g_{ij}$ , are not constant but are functions of the coordinates, then the corresponding space is not ‘flat’ (Euclidean) but ‘curved’. This, finally, is what ‘curved’ means: it is a measure of the departure from Euclidean space, and is determined by certain functions (Riemann’s ‘curvature functions’) of the $g_{ij}$ -coefficients. While the ‘distance’ is specific to the given space, its value is invariant as regards which coordinate representation has been adopted. | ||
+ | |||
+ | <cite>The Lazy Universe by Coopersmith</cite> | ||
+ | </blockquote> | ||
+ | <tabbox Why is it interesting?> | ||
+ | | ||
- | <tabbox Why is it interesting?> | + | The Pythagorean theorem enables us to draw accurate maps. This is achieved by covering the area that should be mapped with a virtual network of triangles. These triangles, via the Pythagorean theorem, allow us to measure distances and angles between stretches of land. This method is known as triangulation. |
- | The Pythagorean theorem enables us to draw accurate maps. | + | It is still used today, for example, to deduce the locations from given satellite data. |
</tabbox> | </tabbox> | ||