Let x, y, and a be sets and X,Y,x',y' be elements.
Denote X *x as X in (is an element of) x, I as intersection, and U as union.
If we can show that for all X *x, X *y (and similarly, if for all Y *y, Y *x), then we are done.
Case 1) xIa is empty
Then x, a and y, a have no elements in common. So, if xUa and yUa are equal, then for all y' *yUa but y' not*a, y' *y. Since xUa and yUa are equal, either y' *a or y' *x. But we supposed y' is not*a, so y'*x. Similarly, for all x' *x, x'*y. QED
Case 2) xIa is non-empty
Define a' as a - {x| x *xIa}. Then xIa' is empty, and you can use the same prove as above, replacing a with a'. QED
Intersection of Medians-Centroid Intersection of Altitudes-Orthocentre
A volume.
To prove that the sum of the angles formed by the intersection of the diagonals within a scalene pentagon equals 180 degrees, you can use the fact that any polygon can be divided into triangles. In a pentagon, there are five sides, and thus it can be divided into three triangles by drawing diagonals. The interior angles of these triangles sum to 540 degrees, and since the angles at the vertices of the pentagon contribute to this sum, the angles formed by the intersection of the diagonals can be shown to sum to 180 degrees by subtracting the angles at the vertices from 540 degrees and considering the properties of linear pairs.
centroid
centroid
Intersection of Medians-Centroid Intersection of Altitudes-Orthocentre
orthocenter
It is the orthocentre.
A volume.
If all three lines are parallel, there are zero points of intersection. If all three lines go through a point, there is one point of intersection. If two lines are parallel and the third one crosses them, there are two. If the three lines make a triangle, there are three points.
To prove that the sum of the angles formed by the intersection of the diagonals within a scalene pentagon equals 180 degrees, you can use the fact that any polygon can be divided into triangles. In a pentagon, there are five sides, and thus it can be divided into three triangles by drawing diagonals. The interior angles of these triangles sum to 540 degrees, and since the angles at the vertices of the pentagon contribute to this sum, the angles formed by the intersection of the diagonals can be shown to sum to 180 degrees by subtracting the angles at the vertices from 540 degrees and considering the properties of linear pairs.
centroid
centroid
The origin.
the orthocenter (:
centroid
Angle a plus angle b subtract from 180 equals angle c