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its simple
as we know
F = dU/dr
F = d(-k/2r^2)/dr
F = k/r^3
as we know F = mv^2/r
mv^2/r = k/r^3
mv^2 = k/r^2
1/2 mv^2 = 1/2 k/r^2
K.E = 1/2 k/r^2
and as given U = -k/2r^2
total energy = K.E + U
total energy = zero