fix(curriculum): rework Project Euler 91 (#42224)

* fix: rework challenge to use argument in function

* fix: add solution

* fix: use MathJax to improve math notation

curriculum/challenges/english/10-coding-interview-prep/project-euler/problem-91-right-triangles-with-integer-coordinates.md

@@ -8,28 +8,46 @@ dashedName: problem-91-right-triangles-with-integer-coordinates
 
 # --description--
 
-The points P (`x`<sub>1</sub>, `y`<sub>1</sub>) and Q (`x`<sub>2</sub>, `y`<sub>2</sub>) are plotted at integer co-ordinates and are joined to the origin, O(0,0), to form ΔOPQ.
+The points ${P}(x_1, y_1)$ and ${Q}(x_2, y_2)$ are plotted at integer co-ordinates and are joined to the origin, ${O}(0, 0)$, to form ${\Delta}OPQ$.
 
 <img class="img-responsive center-block" alt="a graph plotting points P (x_1, y_1) and Q(x_2, y_2) at integer coordinates that are joined to the origin O (0, 0)" src="https://cdn-media-1.freecodecamp.org/project-euler/right-triangles-integer-coordinates-1.png" style="background-color: white; padding: 10px;">
 
-There are exactly fourteen triangles containing a right angle that can be formed when each co-ordinate lies between 0 and 2 inclusive; that is, 0 ≤ `x`<sub>1</sub>, `y`<sub>1</sub>, `x`<sub>2</sub>, `y`<sub>2</sub> ≤ 2.
+There are exactly fourteen triangles containing a right angle that can be formed when each co-ordinate lies between 0 and 2 inclusive; that is, $0 ≤ x_1, y_1, x_2, y_2 ≤ 2$.
 
 <img class="img-responsive center-block" alt="a diagram showing the 14 triangles containing a right angle that can be formed when each coordinate is between 0 and 2" src="https://cdn-media-1.freecodecamp.org/project-euler/right-triangles-integer-coordinates-2.png" style="background-color: white; padding: 10px;">
 
-Given that 0 ≤ `x`<sub>1</sub>, `y`<sub>1</sub>, `x`<sub>2</sub>, `y`<sub>2</sub> ≤ 50, how many right triangles can be formed?
+Given that $0 ≤ x_1, y_1, x_2, y_2 ≤ limit$, how many right triangles can be formed?
 
 # --hints--
 
-`rightTrianglesIntCoords()` should return a number.
+`rightTrianglesIntCoords(2)` should return a number.
 
 ```js
-assert(typeof rightTrianglesIntCoords() === 'number');
+assert(typeof rightTrianglesIntCoords(2) === 'number');
 ```
 
-`rightTrianglesIntCoords()` should return 14234.
+`rightTrianglesIntCoords(2)` should return `14`.
 
 ```js
-assert.strictEqual(rightTrianglesIntCoords(), 14234);
+assert.strictEqual(rightTrianglesIntCoords(2), 14);
+```
+
+`rightTrianglesIntCoords(10)` should return `448`.
+
+```js
+assert.strictEqual(rightTrianglesIntCoords(10), 448);
+```
+
+`rightTrianglesIntCoords(25)` should return `3207`.
+
+```js
+assert.strictEqual(rightTrianglesIntCoords(25), 3207);
+```
+
+`rightTrianglesIntCoords(50)` should return `14234`.
+
+```js
+assert.strictEqual(rightTrianglesIntCoords(50), 14234);
 ```
 
 # --seed--
@@ -37,16 +55,65 @@ assert.strictEqual(rightTrianglesIntCoords(), 14234);
 ## --seed-contents--
 
 ```js
-function rightTrianglesIntCoords() {
+function rightTrianglesIntCoords(limit) {
 
   return true;
 }
 
-rightTrianglesIntCoords();
+rightTrianglesIntCoords(2);
 ```
 
 # --solutions--
 
 ```js
-// solution required
+function rightTrianglesIntCoords(limit) {
+  function isRightTriangle(points) {
+    for (let i = 0; i < points.length; i++) {
+      const pointA = points[i];
+      const pointB = points[(i + 1) % 3];
+      const pointC = points[(i + 2) % 3];
+      const vectorAB = [pointB[0] - pointA[0], pointB[1] - pointA[1]];
+      const vectorAC = [pointC[0] - pointA[0], pointC[1] - pointA[1]];
+
+      if (isRightAngleBetween(vectorAB, vectorAC)) {
+        return true;
+      }
+    }
+    return false;
+  }
+
+  function isRightAngleBetween(vector1, vector2) {
+    return vector1[0] * vector2[0] + vector1[1] * vector2[1] === 0;
+  }
+
+  function getSetKey(points) {
+    return (
+      '0.0,' +
+      points
+        .sort((a, b) => a[0] - b[0])
+        .map(point => point.join('.'))
+        .join(',')
+    );
+  }
+
+  const pointO = [0, 0];
+  const rightTriangles = new Set();
+  for (let x1 = 1; x1 <= limit; x1++) {
+    for (let y1 = 0; y1 <= limit; y1++) {
+      const pointP = [x1, y1];
+      for (let x2 = 0; x2 <= limit; x2++) {
+        for (let y2 = 1; y2 <= limit; y2++) {
+          const pointQ = [x2, y2];
+          if (pointP[0] === pointQ[0] && pointP[1] === pointQ[1]) {
+            continue;
+          }
+          if (isRightTriangle([pointO, pointP, pointQ])) {
+            rightTriangles.add(getSetKey([pointP, pointQ]));
+          }
+        }
+      }
+    }
+  }
+  return rightTriangles.size;
+}
 ```