According to law of cosines the length of RQ can be written as
.
Given the length PR is 6 , the length of RQ is p, the length of PQ is 8 and the angle RPQ is 39 degrees.
A length of the triangle can be written as according to law of cosines if sides are given and one angle is 
We have to just put the values in the above equation.
as
.
p is the side opposite to angle given , the length of other sides are 6 and 8 and angle is 39 degrees.
Hence the side can be written as according to law of cosines if the angle is 39 degrees is as
.
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Answer:
D
Step-by-step explanation:
What don't you get about the problem ?
It is the third answer down
D
is certainly wrong. You could extend the length of AD as far as you want and the two triangles (ABD and ACD) would still be congruent.
C
is wrong as well. The triangles might be similar, but they are more. They are congruent.
B
You don't have to prove that. It is given on the way the diagram is marked.
A
A is your answer. The two triangles are congruent by SAS