(2)|x-1|+|3-x|2.已知|m-n|+|n-9|=0,求m+n的值.

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(2)|x-1|+|3-x|2.已知|m-n|+|n-9|=0,求m+n的值.
(2)|x-1|+|3-x|
2.已知|m-n|+|n-9|=0,求m+n的值.

(2)|x-1|+|3-x|2.已知|m-n|+|n-9|=0,求m+n的值.
15题,1,|x-3|/(x-3)+|x-1|/(x-1)=(3-x)/(x-3)+(x-1)/(x-1)=0
2,|x-1|+|3-x|=x-1+3-x=2
2 |m-n|+ |n-9|=0→m-n=0 ①
,n-9=0 ②
由②得n=9,代入①得m=9