Reputation
Top tag
Next privilege 125 Rep.
Vote down
Badges
7
Newest
 Teacher
Impact
~444 people reached

  • 0 posts edited
  • 0 helpful flags
  • 4 votes cast
Apr
15
comment How to calculate sum of combinations with different n and k
@Sab The identities used are ${n \choose m}={n \choose n-m}$ and ${n \choose m}={n-1 \choose m-1}+{n-1 \choose m}$. You may see here. From the former the answer can be written as ${n+D \choose D}-1$.
Apr
12
comment What's the difference between $\sum_{r=1}^n(ar+b)$ and $\sum_{r=1}^nar+b$
The latter is correct. You can regard $u_r$ as $f(r)$.
Apr
10
revised How to calculate sum of combinations with different n and k
added 16 characters in body
Apr
10
revised How to calculate sum of combinations with different n and k
added 37 characters in body
Apr
10
revised How to calculate sum of combinations with different n and k
added 109 characters in body
Apr
10
comment How to calculate sum of combinations with different n and k
@Sab It's because ${n \choose m}={n \choose n-m}$.
Apr
10
answered How to calculate sum of combinations with different n and k
Apr
9
awarded  Teacher
Apr
9
answered Number of the term
Apr
6
accepted What is the volume of $A'EF-ABD$?
Apr
5
asked What is the volume of $A'EF-ABD$?
Mar
9
accepted Show that $x^2+15y^2=4^n$ has at least $n$ non-negative integer solutions
Mar
8
revised Show that $x^2+15y^2=4^n$ has at least $n$ non-negative integer solutions
added 73 characters in body
Mar
8
comment Show that $x^2+15y^2=4^n$ has at least $n$ non-negative integer solutions
But, for $4^3$ I need $3$ solutions.
Mar
8
comment Show that $x^2+15y^2=4^n$ has at least $n$ non-negative integer solutions
Sorry, I don't understand. It seems that this method can only generate even solutions? I have tried induction and $(x_{k+1}, y_{k+1}) = (2x_k, 2y_k)$.
Mar
8
asked Show that $x^2+15y^2=4^n$ has at least $n$ non-negative integer solutions
Jan
30
accepted Show that $\lim_{n \to \infty }(nx^{n+1}-(n+1)x^n)=0$
Jan
29
asked Show that $\lim_{n \to \infty }(nx^{n+1}-(n+1)x^n)=0$
Jan
20
awarded  Supporter
Jan
20
awarded  Informed