Answer with Explanation:

= 1 (1 + xb/xa + xc/xa ) +  1/ (1 + xa/xb + xc/xb )  +  1 (1 + xb/xc + xa/xc ) 

= xa /(xa + xb + xc) + xb / (xa + xb + xc) + xc / (xa + xb + xc) 

= (xa + xb + xc) / (xa + xb + xc) 

= 1.

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