If 20 × 20 = 400, execute the difference of two squares to find 19×21? I have no idea how to do this. any help would be great please?
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Originally Posted by mathkid182 If 20 × 20 = 400, execute the difference of two squares to find 19×21? I have no idea how to do this. any help would be great please? The difference of two squares is . Let and . Do you see where to go from here?
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
Originally Posted by mathkid182 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 .
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?
I would replace that last line with 399 = 19 x21 instead of 399 = 399. The latter does not actually constitute a proof.
perfect thanks again your a great teacher!!
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