Step 1:multiply by :

.......... to get:

............... ..............................

Step 2: form the ratio: to get,after doing some calculations:

................... .................................................. ......................

Step 3: multiply by:

.............. to get after doing some calculations:

.....................- .................................................. ...................

Step 4: let h go to zero to get:

...................-1/2 ,which is the desired limit............................................. .....................................