Answer:
(a) 54.1°
Explanation:
The Law of Cosines can be used to find the angle measure when three sides lengths are given.
j² = k² +l² -2kl·cos(J)
J = arccos((k² +l² -j²)/(2kl)) = arccos(789/1344) ≈ 54.0518°
The measure of angle J is about 54.1°.
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Additional comments
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As a rough approximation of the angle measures, you can assume they are nearly proportional to the side lengths. Then angle J would be ...
J/180° = j/(j+k+l) = 26/(26+32+21) = 26/79
J ≈ 180°×26/79 ≈ 59.2°
This is sufficient to tell you the angle is not 40° and not 85°.