Question 834485
(a-bi)(a+bi)


a(a+bi)-bi(a+bi)


a(a)+a(bi)-bi(a)-bi(bi)


a^2+abi-abi-(bi)^2


a^2+abi-abi-b^2(i^2)


a^2+abi-abi-b^2(-1)


a^2+abi-abi+b^2


a^2+b^2


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So,


(a-bi)(a+bi) = <font color="red">a^2+b^2</font>