Can anyone please help

redmons2

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Oct 26, 2014
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Question.jpg

Its question C (i)

I have attached the answer too. Could anyone please tell me were the 102x comes from in the answer ? I understand the rest of the sum. Thank you

answer.jpg
 
Surely you know that \(\displaystyle (a+ b)^2= a^2+ 2ab+ b^2\)? Here, you have \(\displaystyle (17- 3x)^2= (17)^2+ 2(17)(-3x)+ (-3x)^2= 289- 102x+ 9x^2\) because 2(17)(3)= 102.
 
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