



 
Hi Eric, Let me use $f(x) = x^{1/2} = \large \frac{1}{x^{1/2}}.$ You need to work with \[f(x + h)  f(x) = \frac{1}{(x + h)^{1/2}}  \frac{1}{x^{1/2}}.\] Using the common denominator of $ x^{1/2} \times (x + h)^{1/2}$ and simplifying I get \[f(x + h)  f(x) = \frac{x^{1/2}  (x + h)^{1/2}}{x^{1/2} \times (x + h)^{1/2}}.\] At this point multiply the numerator and denominator by $x^{1/2} + (x + h)^{1/2},$ use the fact that you have a difference of squares in the numerator and simplify. See if you can complete the derivation from here. Write back if you need more assistance,  


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