4i(4+9i) write each expression as a complex number in the form a+bi

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The letters a and b are the real numbers and the letter i is the imaginary part.  The letter i, when squared, equals as you know, -1.  You can use the distributive rule to do away with the parentheses.  4i(4 + 9i) = 4i * 4 = 16i 4i * 9i = 36i^2 36i^2 = 36 * (-1) = -36 -36 + 16i is now in the form a + bi, where a = -36 and b = 16.