I know why the guest got this question incorrect. Let's assume that m∠ADC=90∘, thus making it a right triangle. Let's make the assumption, too, that AD=DC, thus making it an isosceles triangle.
(4√2)22 | This utilizing the triangle area formula. |
16∗(√2)22 | |
8∗√22 | |
8∗2 | |
16units2 | |
However, there is a key word. That word would happen to be "altitude." An altitude is a perpendicular height that extends from a vertex to the opposite side. Notice how the height of this triangle does not do this. Therefore, the original diagram does not fit the original criteria.
This is a sketch of the given info. ¯AD is an altitude and ¯AC≅¯AB, and the triangle is right.
Because ¯AC≅¯AB by the given info, the isosceles triangle theorem tells us that the angles opposite of their sides are congruent, so ∠B≅∠C. We can now figure out m∠Bandm∠C.
m∠B+m∠C+m∠BAC=180∘ | Now, substitute in the known values. |
m∠B+m∠C+90=180 | m∠B=m∠C |
m∠B+m∠B+90=180 | Now, solve for both angles. |
2m∠B+90=180 | |
m∠B+45=90 | |
m∠B=45∘ | |
m∠B=m∠C=45∘ | |
Since an altitude always bisects the angle of an isosceles triangle, m∠CAD=12∗90=45∘.
Since m∠CAD=m∠C=45∘, by the inverse of the isosceles triangle theorem, AD=DC=4√2
The entire base, however, is 2 times this length, so BC=2∗4√2=8√2. The altitude is 4√2. Now, let's use the area formula for a triangle.
12bh=12(8√2)(4√2) | Now, simplify here. |
12(32∗(√2)2) | |
12(32∗2) | The 1/2 and 2 cancel out here. |
32 | This is the area of the triangle. |
The length of ¯AC can be found using the law of cosines. The law of cosines relates the remaining side by knowing two sides and its opposite included angle. I think this picture sums it up nicely.
Knowing this information, we can now find the missing length.
c2=a2+b2−2abcosC | Now, plug in the values we already know in the diagram and solve for the missing side. |
AC2=12+32−2(1)(3)cos40∘ | Now, take the principal square root of both sides since the negative answer is nonsensical in the context of geometry. |
AC=√12+32−2(1)(3)cos40∘≈2.32 | No units are given in the problem. |