`|x-y|=13+|-25|`
`⇒|x-y|=13+25`
`⇒|x-y|=38`
`⇒`\(\left[ \begin{array}{l}x-y=38\\x-y=-38\end{array} \right.\)
`⇒`\(\left[ \begin{array}{l}x=y+38\\x=y-38\end{array} \right.\)
`1-5+9-13+…-x=400`
`⇒(1-5)+(9-13)+...+(x-4-x)=400`
`⇒(-4)+(-4)+...+(-4)=400 ` Có `1/2.[(x+3)/4]` số`(-4)`
`⇒1/2.(-4). (x+3)/4 =400`
`⇒x=797`
Vậy `x=797`