fix the relative tolerance

sandbox/equations.ipynb

@@ -137,7 +137,7 @@
     "So if we inject this back in the equation above, we get:\n",
     "\n",
     "$$\n",
-    "\\sigma_F^2 \\approx \\sum_{i=1}^{n} \\left( \\frac{\\partial F}{\\partial x_i} \\frac{x_i}{F(x_1, x_2, ..., x_n)} \\right)^2 \\sigma_{x_i}^2\n",
+    "\\frac{\\sigma_F^2}{F(x_1, x_2, ..., x_n)} \\approx \\sum_{i=1}^{n} \\left( \\frac{\\partial F}{\\partial x_i} \\frac{x_i}{F(x_1, x_2, ..., x_n)} \\right)^2 \\frac{\\sigma_{x_i}}{x_i}^2\n",
     "$$\n",
     "\n",
     "## Cost function\n",