we start at AI started at the corner opposite A, at the other end of the diagonal of the parallelogram, in order to end up at A
there is a plus sign and not a minus signCosines of supplementary angles have opposite sign
"parallel to the diagonal".The translational position of a vector in a force diagram is irrelevant (unless you're dealing with torques)
length of ADI take it that D is now our label for the corner of the parallelogram opposite A,
The [square of the] length of the diagonal AD is b²+c²-2bc.cos(A)Are we using different D?
an angle like B+C does not occurAn angle with magnitude B+C occurs in the parallelogram of which AB abd AC are adjacent sides.