Which values of x and y would make the following expression represent a real number?(4 + 5i)(x + yi)

Multiplying gives (4 + 5i)(x + yi) = (4x − 5y) + (4y + 5x)i. For the result to be real, the coefficient of i must be 0. With x = 4 and y = −5, 4(−5) + 5(4) = 0, so choice C, x = 4 and y = −5, makes the expression real.

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