= 2∠AMP.
non-zero digits such that deleting consecutively the digit of the left, in each step, we obtain a divisor of the previous number.
Let ![$[n]$](http://latex.artofproblemsolving.com/0/d/e/0deff9086fd7840ef23860f5bc924f1d4d2d2310.png)
denote the set of natural numbers less than or equal to 
![$f(m,n) = \sum_{(x_1,x_2,x_3, \cdots, x_m) \in [n]^{m}} \frac{x_1}{x_1 + x_2 + x_3 + \cdots + x_m} \binom{n}{x_1} \binom{n}{x_2} \binom{n}{x_3} \cdots \binom{n}{x_m} 2^{\left(\sum_{i=1}^{m} x_i\right)}$](http://latex.artofproblemsolving.com/f/d/f/fdf7b2fcfb960e93d59f3f84cef4372262c69499.png)
Y by Implicit25, Adventure10, Mango247
Points 
and 
are chosen on the base side 
and the lateral side 
of an isosceles trapezoid 
, respectively. Quadrilateral 
is an isosceles trapezoid as well. Prove that 
.
and 
are chosen on the base side 
and the lateral side 
of an isosceles trapezoid 
, respectively. Quadrilateral 
is an isosceles trapezoid as well. Prove that 
.
