Posted by velagala on Saturday, February 4, 2012 at 11:05am.
8
The fact that it is a 13 by 13 is irrelevant.
Draw your 2 diagonals and count the number of points you see. Label the points A, B, C, D, and E, where E is the intersection of the diagonals.
A triangle is formed by connecting any 3 points, unless the 3 points lie in a straight line
e.g. ABC will form a triangle
AED will form a triangle , but AEC will not
So it becomes the number of ways we can choose 3 of the 5 points, which is
C(5,3) = 5!/(3!2!) = 10
but that would include AEC and BED, which form a straight line,
so there are 10-2 or 8 triangles
You could use the above reasoning for any polygon
Related Questions
geometry - 1. An exterior angle of a regular polygon measures 30 degrees. What ...
math - These numbers form an addition and subtraction fact family. 6, 7, 13 ...
Geometry - 1. How many sides does a regular polygon have if one exterior angle ...
Math - If I have ((square root of 13)+(6))^2 would it be factored as -((square ...
Trigonometry - these values right? tan theta=2/3 sec theta=Squareroot(13)/3 sine...
geometry - If all of the diagonals are drawn from a vertex of a quadrilateral, ...
Geometry - The diagonals of rectangle ABCD are AC and BD. Hallie found that the ...
algebra - can someone tell me if i did the problem correct (solve by completing ...
algebra - solve and check.label ant contradictions or identities. 5(t+1)+8=3(t-2...
Math - How do you find a square root of a number that's not a perfect square...
For Further Reading
