Physics (A man in the lift)

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A man of mass m is standing in an elevator. The elevator is accelerating downward at g/k, where k>1 and g is the gravitation acceleration. If the force on the man by the elevator's floof jmg, where j is a constant and k= a/(b+cj), where gcd(|a|,|b|,|c|)=1, what is a+b+c? If k cannot be expressed in this form, say so.

• Physics (A man in the lift) -

mg down
F up
mg - F = m a = m g/k

mg - jmg = (mg/a)(b+cj)

1-j = (b+cj)/a = 1/k

k (1-j) = 1

I do not know what gcd(|a|,|b|,|c|)=1 means so can go no further.

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