If 1a(b+c),1b(c+a),1c(a+b) be in H.P., then a,b,c are also in H.P.
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Solution
Taking reciprocals we are given that a(b + c), b(c + a), c(a + b) are in A.P. Subtract ab + bc + ca from each -bc, -ca, -ab are in A.P. Divide by - abc, or 1a,1b,1c are in A.P. or a,b,c are in H.P.