[د.امير]

Dana,
simplify the drawing as shown in the attachment. Then you will see simple diagram
so, the resultant force, R, should be
R^2=F1^2 + F2^2 -2xF1xF2Cos (180-30
or
R^2=10000 +10000 -2 x 100 x 100 x cos 150)
[OVERLINE]R = 193N answer[/OVERLINE]
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Note: you will see that when the angle is 90
cos 90 is zero
so the eqation above will revert back to the form you know, which is
R^2=F1^2 + F2^2 special case
However, the general form of the equation is
R^2=F1^2 + F2^2 -2xF1xF2Cos (alpha)