Update 693-math-deriv.markdown

Author: Joe7M

_build/reference/693-math-deriv.markdown

@@ -5,10 +5,45 @@
 Calculation of derivative. errcode = 0 for success; otherwise calculation error. 
 
 
-* x value of x
-* maxtries maximum number of retries
-* maxerr tolerance
-* errcode 0 for success; otherwise calculation error
-* result the result
+* x: value of x
+* maxtries: maximum number of retries
+* maxerr: tolerance
+* errcode: 0 for success; otherwise calculation error
+* result: the result
+* expr: f(x)
 
+```
+' What is the derivative of f(x) for a given x
 
+def f(x) = 2*x^2+2
+
+x = 3
+result = 0
+errcode = 0
+
+deriv x, 10, 0.01, result, errcode USE f(x)
+
+
+def d(x) = 4*x ' Exact derivative of f(x) for comparision
+
+print "Result of DERIV  at x = "; x; " : "; result
+print "Exact derivative at x = "; x; " : "; d(x)
+```
+
+```
+' What is the derivative of f(x) for a given x
+
+def f(x) = x^3 + 2 * exp(2*x)
+
+x = 3
+result = 0
+errcode = 0
+
+deriv x, 10, 0.000001, result, errcode USE f(x)
+
+
+def d(x) = 3*x^2 + 4*exp(2*x) ' Exact derivative of f(x) for comparision
+
+print "Result of DERIV  at x = "; x; " : "; result
+print "Exact derivative at x = "; x; " : "; d(x)
+```