add exponential function

src/exponent.c

@@ -39,3 +39,11 @@ unsigned long long fac(int x){
     return prod;
 }
 
+
+double exponential(double x){
+    double sum = 0.0;
+    for(int i = 1; i<=21; i++){
+        sum += (1/fac(i)*(expIN(i,x)));
+    }
+    return sum;
+}

src/exponent.h

@@ -3,5 +3,6 @@
 
 double expI(double e, double num);
 double expIN(double e, double num);
+double exponential(double x);
 
 #endif // exponent.h