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Complete the parallogram and draw in the resultant, which would be the diagonal.
R^2 = 3^2 + 8^2 - 2(3)(8)cos 120° = 97
|R| = √97 = appr 9.85 N
or , place one vector along the x-axis, ....
|R| = 3(cos0, sin0) + 8(cos60,sin60)
= <3,0> + <4,6.928..>
= <7,6.928>
= √(49 + 48)
= √97