22.- Law of cosines.
In a right triangle we can calculate the c side using the Pythagorean theorem.
If the triangle is not a rectangle, we can calculate the c side using the Law of Cosines.
apk file in external link: http://kio4.com/appinventor/apk_archivos.htm
teorema_cosenos_i.aia (3.6 KB) (unfinished)
23.- Exponential regression.
Create an application to obtain the expression of an ab-exponential and e-exponential regression.
- ab-Exponential regression. 
You can get information about the ab-exponential regression in:
- e-Exponential regression. 
You can get information about the e-exponential regression in:
apk file in external link: http://kio4.com/appinventor/apk_archivos.htm
p84F2_regresion_exponencial_i.aia (30.2 KB) (unfinished)
@Juan_Antonio outstanding job with these. Please continue to share. AI2 community is very lucky to have you contributing work of your quality. God bless 
Hi, with this apk is possible calculate a integral of a real number? for example int(0,74)? tks
This is an integral
24.- Calculate PI.
Create an app to calculate the value of PI with many decimals.
The output should be a decimal sequence beginning 3.14159265 ...
25.- Find the division between two numbers by repeatedly subtracting the dividend minus the divisor.
Example: 22 / 4
hello, great demonstration !!!
if I take your first example on calculating the area of a polygon, is it possible to color it?
what is the method (I am not as efficient as you in this field, I would even say beginner :-))
Thanks for reading
perhaps see here:
A post worth bookmarking, not just for App Inventor but also if you need help with Maths 
26.- Cistercian Numerals.
Write a decimal number from 1 to 9999 and convert it into a Cistercian number.
Cistercian number: http://www.davidaking.org/Ciphers.htm
p91G_cisterciensesi.aia (19.3 KB)
[not finished. It contains the 36 images of the symbols, with a transparent background.]
Did you watch this? 
27.- Find the crossing point of two lines perpendicular to two sides of an equilateral triangle.
apk file in external link: http://kio4.com/appinventor/apk_archivos.htm
We have an equilateral triangle whose sides are 100.
On the left side we draw a segment of 65 and on the right side another of 47.
Find the equation of the line that passes through these points and is perpendicular to the sides.
Find the point where these lines intersect.
This triangle is used in various problem domains, from describing earth samples based on their proportions of sand, clay, and silt (a recent question on this forum) to choosing newly created character attribute values in the game Fallout 4 (according to my son (I haven't reached that game yet.))
The equilateral triangle has the interesting property that any point within the triangle's closest distances to the the three sides always add up to the same number, the height of the triangle.
Unfortunately, no source code is here, so we have to derive the equations from scratch.
28.- Mathematical proposal for Pi day.
A circle has radius 7. We draw a radius at 30º and another at 31º, as the schematic figure indicates.
a) Find the value of the segment s.
b) What is the sum of the 360 segments s drawn on the circle?
c) What value do we obtain by dividing the previous sum by the diameter of the circle?
d) Carry out the previous sections when the radius are 0.1º apart.
29.- Abacus Clock.
This "Abacus Clock" application indicates the actual time: HHMMSS.
It is a good idea to do it with App Inventor.
https://apkpure.com/clock-abacus/appinventor.ai_mydigitaldesk.clock_abacus
30.- Validate Credit Card.
Write an application where the user enters a credit card number and shows if that card is valid or not.
Use the Luhn algorithm:
http://www.brainjar.com/js/validation/default2.asp
Can you please tell where can I download extension?
