Answer:
71.6°
Explanation:
The angle can be found from the dot product of PQ and PR.
PQ·PR = |PQ|×|PR|×cos(α)
where α is the angle between the two segments.
cos(α) = (Q -P)·(R -P)/(|Q -P|×|R -P|)
= ((0, 1, 0) -(1, 0, 0))·((0, 0, 2) -(1, 0, 0))/(|Q -P|×|R -P|)
= (-1, 1, 0)·(-1, 0, 2)/(√(((-1)² +1²)((-1²) +2²)) = (1+0+0)/√10
α = arccos(1/√10) ≈ 71.6°
The angle at P is about 71.6°.
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Additional comment
The side lengths of the triangle are √2, √5, √5. As we have seen, the angle at P is bounded by the sides of length √2 and √5. The law of cosines can also be used to arrive at the angle between these sides.