Toan 8 hang dang thuc

P

pinkylun

$(a^2+b^2)(c^2+d^2)$

$=a^2c^2+a^2d^2+b^2c^2+b^2d^2$

$=a^2c^2+2abcd+b^2d^2+a^2d^2-2abcd+b^2c^2$

$=(ac+bd)^2+ (ad-bc)^2$ (đpcm) :D
 
V

vanmanh2001


1.Rút gọn $(x-1)(x+4)(x-3)-(x+1)(x-4)(x+3)$
$= (x^3 - 13x + 12) - (x^3 - 13x - 12)$
$= 24$
$ 2) (a^2+b^2)(c^2+d^2)=(ac+bd)^2+ (ad-bc)^2$
$(a^2+b^2)(c^2+d^2) = (ac)^2 + 2abcd +(bd)^2 + (ad)^2 -$ $2abcd + (bc)^2$
$(a^2+b^2)(c^2+d^2) = c^2(a^2+b^2) + d^2(a^2+b^2)=$ $(c^2+d^2)(a^2+b^2) (đpcm)$
P/s: bạn Pink nhanh quá :D
 
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