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Math Help - help please-hard question

  1. #1
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    help please-hard question

    If 20 20 = 400, execute the difference of two squares to find 1921?

    I have no idea how to do this. any help would be great please?
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  2. #2
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    Re: help please-hard question

    Quote Originally Posted by mathkid182 View Post
    If 20 20 = 400, execute the difference of two squares to find 1921?

    I have no idea how to do this. any help would be great please?
    The difference of two squares is x^2-y^2=(x-y)(x+y). Let x=20 and y=1. Do you see where to go from here?
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  3. #3
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    Re: help please-hard question

    not really im sorry, where did the 20 and the 1 come from? and what would be the next step?
    would it be

    20^2 - 1^2 = (20-1)(20+1)
    400 - 1= 400+20-20-1
    399 = 399
    ?thanks
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  4. #4
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    Re: help please-hard question

    Quote Originally Posted by mathkid182 View Post
    not really im sorry, where did the 20 and the 1 come from? and what would be the next step?
    would it be

    20^2 - 1^2 = (20-1)(20+1)
    400 - 1= 400+20-20-1
    399 = 399
    ?thanks
    The 20 and the 1 basically came from the question. The point being 400-1=x^2-y^2=(x-y)(x+y)=(20-1)(20+1)= 19 \times 21.
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  5. #5
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    Re: help please-hard question

    Oh i think i get it now thank you so much. do think it would be fine to write the answer as:

    =x2 - y2 = (x-y)(x+y)
    =20^2 - 1^1 = (20-1)(20+1)
    =400-1 = (20-1)(20+1)
    =399 = 399

    therefore 19x21 = 399

    and that would be right?
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  6. #6
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    Re: help please-hard question

    I would replace that last line with 399 = 19 x21 instead of 399 = 399. The latter does not actually constitute a proof.
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  7. #7
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    Re: help please-hard question

    perfect thanks again your a great teacher!!
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