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T

thangvegeta1604

1) a. $(x+y+z+t)(x+y-z-t)=[(x+y)+(z+t)].[(x+y)-(z+t)]=(x+y)^2-(z+t)^2$.
b. $(x-y+z-t)(x-y-z+t)=[(x-y)+(z-t)].[(x-y)-(z-t)]=(x-y)^2-(z-t)^2$.
 
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