Question

Using the law of cosines, in triangle DEF, if e=18 yd, d= 10 yd, f= 22yd, find angle D.

Answer 1

We are given SSS
this means we are given all 3 sides
this means we indeed have to use the law of cosines as suggested.

Pretend we have a triangle with the following labels for the angles of that triangle: ABC (in that order that they occur)

The sides being a,b, and c.
Where a is opp of A
where b is opp of B
where c is opp of C
So the law of cosines for it would be

a^2=b^2+c^2-2bccos(A)
b^2=a^2+c^2-2accos(B)
c^2=a^2+b^2-2abcos(C)