The law of cosines can be helpful here. The largest angle (C) will be opposite the longest side (c). That law tells us ...
... c² = a² + b² -2ab·cos(C)
Then
... cos(C) = (a² +b² -c²)/(2ab)
We don't actually need the angle. We only need to know the sign of the cosine.
... cos(C) = (7² +15² -17²)/(2·7·15) = -15/210 . . . . negative sign, so C > 90°
The sail has the shape of an obtuse triangle.
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A triangle solver app on your calculator, phone, tablet, or computer can give you the result easily.
