How do you simplify $(2 + 5i)(4 - i)$ ?

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Answer 1 · 2016-06-28T23:55:15

$13 + 18i$

Foil the brackets, so we end up with:

$2*4 - 2*i + 5i*4 + 5i*(-i)$

Remember that $i^2 = -1$

so we have:

$8 - 2i + 20i - 5i^2 = 13 +18i$

How do you simplify (2 + 5i)(4 - i)(2 + 5i)(4 - i)?