diff --git a/src/control/double.cc b/src/control/double.cc
index bb9dba1..9cf94db 100644
--- a/src/control/double.cc
+++ b/src/control/double.cc
@@ -24,15 +24,11 @@ DoubleForLoop::SumSquare(int n) {
  *
  * @param n
  * @return the sum of all triangle numbers from T(1) to T(n)
- */
+ */ 
 long
 DoubleForLoop::SumTriangle(int n) {
-  long sum = 0;
-  for (int i = 0; i < n + 1; i += 1) {
-    for (int j = 0; j < i; j += 1) {
-      sum = sum + (long) j;
-    }
-  }
+  if (n <= 0) return 0;
+  long sum = (long)n * (n - 1) * (n + 1) / 6;
   return sum;
 }
 
@@ -98,4 +94,4 @@ DoubleForLoop::SumMatrix(std::vector<std::vector<int>> matrix) {
     }
   }
   return sum;
-}
+}
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