Question

Which characteristics will prove that ΔDEF is an acute, isosceles triangle?

a. segment DE and segment EF are congruent to each other but not to segment DF, and their slopes are not related.
b.segment DE and segment EF are congruent, and their slopes are opposite reciprocals.
c. segment DE is larger than segment EF, and their slopes are not related.
d. segment DE is larger than segment EF, and their slopes are opposite reciprocals.

Answer 1

Answer is A got it right on the test.

Answer 2

a. segment DE and segment EF are congruent to each other but not to segment DF, and their slopes are not related.

Explanation:

In order for a triangle to be isosceles, two of the line segments must be congruent. (eliminates choices C and D)

If the slopes of the congruent segments are negative reciprocals of each other, the triangle is a right triangle, not an acute triangle. (eliminates choice B)

With choices B, C, D eliminated, we are left with choice A.

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However, that necessary description is not sufficient to constrain the triangle to be an acute triangle. In order for the triangle to be acute, the third side must be less than √2 times the length of either of the congruent sides.

The correct answer is "none of the above."