Concept:Use the algebraic identity (x+y)2=x2+y2+2xy to relate the given expressions and solve for xy.Explanation:Given x+y=10, so (x+y)2=100. Also x2+y2=58.Substitute into the identity: 100=58+2xy.Subtract 58 from both sides: 42=2xy.Divide by 2: xy=21.Answer:xy=21, which matches option A.