thank you daydream ^^ it's an interesting problem !
and about my first question
when there are different letters , it doesn't mean 'different' numbers ?
there is a rule , right ?
In rectangular coordinate system , is (r,s) and (u,v) equidistant from the origin ?
1)r+s=1
2)u=1-r and v=1-s
the An is C but MINE is (E)
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由(2): u^2+v^2=(1-r)^2+(1-s)^2 = r^2+s^2-2(r+s)+2
由(1): r+s=1 ==> u^2+v^2= r^2+s^2
所以答案为: C
In rectangular coordinate system , is (r,s) and (u,v) equidistant from the origin ?
1)r+s=1
2)u=1-r and r=1-s
the An is C but MINE is (E)
from the both of the two prerequisites we get r+s=1 and u+v=1 means (r,s) and (u,v) are both in the line x+y=1
but to get the result ,r^2 +s^2=u^2+v^2 is required , isn't it ?