SOLUTION: x+xy+1=16 x-y=1, then x+y=?

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Question 632043: x+xy+1=16
x-y=1,
then x+y=?
Answer by Edwin McCravy(20063) About Me  (Show Source):
You can put this solution on YOUR website!
eq. 1     x+xy+1=16 
eq. 2        x-y=1,

Solve eq. 2 for x:

eq. 2        x-y=1
eq. 3          x=1+y

Substitute 1+y for x in eq. 1:

eq. 1     x+xy+1=16
  (1+y)+(1+y)y+1=16
      1+y+y+y²+1=16
         y²+2y+2=16
        y²+2y-14=0

y+=+%28-%282%29+%2B-+sqrt%28+%282%29%5E2-4%2A%281%29%2A%28-14%29+%29%29%2F%282%2A%281%29%29+
y+=+%28-2+%2B-+sqrt%284%2B56+%29%29%2F2+
y+=+%28-2+%2B-+sqrt%2860%29%29%2F2+
y+=+%28-2+%2B-+sqrt%284%2A15%29%29%2F2+
y+=+%28-2+%2B-+2sqrt%2815%29%29%2F2+
y+=+%282%28-1+%2B-+sqrt%2815%29%29%29%2F2+
y+=+%28cross%282%29%28-1+%2B-+sqrt%2815%29%29%29%2Fcross%282%29+
y+=+-1+%2B-+sqrt%2815%29%29+

Two solutions for y:

y+=+-1+%2B+sqrt%2815%29%29+ and y+=+-1+-+sqrt%2815%29%29+

Use eq. 3 with first solution for y          

     x=1+y                                
     x=1+-1+%2B+sqrt%2815%29%29+               
     x=sqrt%2815%29                       

First solution:
then x+y=sqrt%2815%29+(-1+%2B+sqrt%2815%29%29+)
then x+y=-1%2B+2sqrt%2815%29%29+


Use eq. 3 with second solution for y          

     x=1+y                                
     x=1+-1+-+sqrt%2815%29%29+               
     x=-sqrt%2815%29                       

Second solution:
then x+y=-sqrt%2815%29+(-1+-+sqrt%2815%29%29+)
then x+y=-1-+2sqrt%2815%29%29+

Edwin