Verified Answer :

bcalle

a^2 + b^2 < c^2 for the triangle to be obtuse
2^2 + 4^2 < x^2
4 + 16 < x^2
20 < x^2
sqrt 20 = 4.7 approx 5
x needs to be greater than or equal to 5 for the triangle to be obtuse.
4 + 16 < 25
20 < 25

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