Question

The solutions to the equations $z^2 = 4 + 4\sqrt{15}\,i$ and $z^2 = 2 + 2\sqrt{3}\,i,$ where $i = \sqrt{-1},$ form the vertices of a parallelogram in the complex plane. The area of this parallelogram can be written in the form $p\sqrt{q} - r\sqrt{s},$ where $p,$ $q,$ $r,$ and $s$ are positive integers and neither $q$ nor $s$ is divisible by the square of any prime number. What is $p + q + r + s?$

$20$

$21$

$22$

$23$

$24$

Answer: A

Concepts:complex number shoelace formula

Difficulty rating: 2270

Solution:

Writing $z = a + bi$ with $(a + bi)^2 = 4 + 4\sqrt{15}\,i$ gives $a^2 - b^2 = 4$ and $2ab = 4\sqrt{15}.$ Then $a^4 - 4a^2 - 60 = 0,$ so $(a^2 - 10)(a^2 + 6) = 0,$ yielding $a = \pm\sqrt{10},$ $b = \pm\sqrt{6}.$ The vertices from the first equation are $\pm(\sqrt{10} + \sqrt{6}\,i).$ The same method on $z^2 = 2 + 2\sqrt{3}\,i$ gives $\pm(\sqrt{3} + i).$

Applying the shoelace formula to $(\sqrt{10}, \sqrt{6}),$ $(\sqrt{3}, 1),$ $(-\sqrt{10}, -\sqrt{6}),$ $(-\sqrt{3}, -1)$ gives area $6\sqrt{2} - 2\sqrt{10}.$ Thus $p + q + r + s$ $= 6 + 2 + 2 + 10$ $= 20.$

Thus, the correct answer is A.

2018 AMC 12A Problem 22

Answer: A

Difficulty rating: 2270

Solution:

Problem 22 in Other Years