(a + b + c)2 = a2 + b2 +c2 + 2(ab + bc + ca)
বা, (15)2 = 83 + 2(ab + bc + ca)
বা, - 2(ab + bc + ca) = 83 - 225
বা, - 2(ab + bc + ca) = - 142
∴ ab + bc + ca=71
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