The vertices of triangle ABC are A(2,3), B(–1,2), and C(3,1). If ΔDEF ∼ ΔABC and DE = 4AB, what is the length of EF?

Since ΔDEF ∼ ΔABC, the vertex order gives EF corresponding to BC and DE corresponding to AB. The side BC has length √((3 − (−1))² + (1 − 2)²) = √17. Because DE = 4AB, the scale factor from ΔABC to ΔDEF is 4, so EF = 4√17, choice D.

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