# TrigonometryTriangle question

#### Bushy

##### Member

In square ABCD need to show $cos a = \frac{1}{12}$

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#### MarkFL

Staff member
While I haven't worked this problem yet, I am thinking the law of cosines may help, along with the relationship between supplementary angles.

#### Fantini

I gave it some thought, and I believe applying law of cosines twice will do the trick. First you apply to the triangle $AEC$ to find side $EC$, and then apply it to the triangle $CED$ to find $\cos a$.