Continued Fraction with 1’s and 3’s

What is the value of this fraction that continues forever in the same pattern? (You’ll want the quadratic formula.)




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So x 1
1 +  1
3 + x

x 1  =  1  =  1  =  x + 3
1 +  1   3 + x + 1     x + 4   x + 4
3 + x 3 + x 3 + x  

x(x + 4) = x + 3
x2 + 4x = x + 3
x2 + 3x – 3 = 0

x -3 ± √9 + 12  =  -3 ± √21 .
2 2

Since x is clearly positive, we go with
-3 + √21  ≈ 0.79
2

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