Why Does the Binomial Theorem Solution Differ from the Book's Answer?

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SUMMARY

The forum discussion centers on the discrepancies between a student's solution to a binomial theorem problem and the answer provided in their textbook. The student is attempting to expand the expression (x+y)n using the binomial coefficient formula C(n,r) = n!/r!(n-r)!, but struggles to incorporate the values of x and y correctly. Key insights include the realization that the sum of n elements can be interpreted as nx, and the importance of manipulating binomial coefficients to derive the expected value of successes in independent trials.

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  • Familiarity with binomial coefficients and factorials
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  • #31
Thanks, this ended up helping a lot =)

and sorry for digging up an old thread!
 
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  • #32
gaganspidey said:
This is what I've been able to work out :

\sum_{r=0}^{n} r C^n_r x^r y^{n-r} <--------(given)

= \frac{n!}{(r-1)!(n-1-(r-1)!} x^r y^{n-r} <---------(cancelling the r's & adding & subtracting 1 & regrouping them)

= \frac{(nx)}{x} \frac{(n-1)!}{(r-1)!(n-1-(r-1)!} x^r y^{n-r} <-------(separating n & multiplying & dividing by x)

= (nx) C^{n-1}_{r-1} x^{r-1}y^{n-r} <-------( The R.H.S)

= nx(y+x)^{n-1} <--------(The L.H.S)

= nx <--------(Substuting the value x+y=1)


I think something's not right above, but what is it ?

Hmm, just a question: The solution seems impeccable, but what happened to the sigma notation after the first line?
 

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