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So, I came across a problem in my math book:

Find the coefficient of y^4 in the expansion of (2y - 5)^7

But I don't know how to do this correctly. My teacher looked over it and gave me a hint to solve the equation first, but there is no equation! I don't know how to get this, please, help. Thank you so much in advance!!!! I really do appreciate the help! 

 Jul 15, 2020
 #1
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Find the coefficient of y^4 in the expansion of (2y - 5)^7  

 

I couldn't expand that by hand, so I went to a math website.  The answer I got there was

 

128y7 − 2240y6 + 16800y5 − 70000y4 + 175000y3 − 262500y2 + 218750y − 78125 

 

So what is the coefficient of y4 according to that?   Looks like it's  –70,000       

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 Jul 15, 2020
 #2
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Thank you so so so much! May I know what math website you used? Thank you again!

 Jul 15, 2020
 #3
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I just asked google search to expand that expression.  The top result was 

 

https://www.mathway.com/popular-problems/Algebra/218494  

 

When the page opened there was an example (x–2y)5 on the screen.  I just clicked on each term and changed them to (2y–5)7 and then clicked the arrow out on the far right of that line.   

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 Jul 15, 2020
 #4
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By "top result" I didn't mean the best one.  I meant it was the top one - the first one - on the list of search results. 

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Guest Jul 15, 2020

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