What's the potential energy?

Question

A force of 10 N stretches a spring that has a spring constant of 20 N/m. What's the potential energy stored in the spring?

 

Answer: 2.5 J

Answer 1

F = kx => 10 = 20x => x = 1/2

Ep = (1/2)kx^2 = (1/2)(20)(1/2)^2 = 2.5 J  <==Answer

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Attn: Here I just used absolute values for all variables.