SOLUTION: What is the measure of the third sides of a triangle, that is already measured with 5meters and 4meters?

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There is no answer to this question because the triangle could any shape.
If the angle between the two known sides was 1 degree, then the third side would relatively short.
If the angle between the two known sides was 179 degrees, then the third side would be relatively long.
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If you have studied the law of cosines, you know that it extends the Pythagorean Theorem to any angle.
c^2 = a^2 + b^2 - 2*a*b*cos(x), where x is the angle between the known sides a and b.
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(I used 'x' where most texts would say the angle was a Greek letter.
But I don't have a Greek letter to type into this reply. Sorry.)
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To use the law of cosines, you have to know the measure of the angle.
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cos 1° = 0.99984769515639
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c^2 = 4^2 + 5^2 - 2*(4)*(5)*(0.99984769515639)
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c^2 = 1.00609219
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c = sqrt(1.00609219) = 1.00304147
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VS.
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cos 179° = -0.99984769515639
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c^2 = 4^2 + 5^2 - 2*(4)*(5)*(-0.99984769515639)
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c^2 = 80.9939078
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c = sqrt(80.9939078) = 8.99966154
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Done.