## Question regarding Binomial expansion.

I have my question and my problem in the attachment that followed.
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 Recognitions: Gold Member Homework Help Science Advisor By "fourth root of 19/21", they mean the quantity $(19/21)^{1/4}$. Like in your other question, you would like to find the x meeting the condition "x^3 is ignorable" and such that $$\frac{1-x}{1+x}=\frac{19}{21}$$ You will find that x easily by solving this last equation for x. Then, with n=1/4, your polynomial formula gives you the wished approximation.
 Thanks for your help.I think I now can understand with what the question want.