(1) If the hook codes are all the same, the number of hook codes represents the magnitude of the force, and the three sides of the triangle represent the magnitude of the three forces, which should form a triangle.

A. When the number of hook codes is N 1=N2=2 and N3=4, because there must be an included angle between N 1N2, A does not hold, so A is wrong.

B when the hook number N 1=N2=N3=4, the two forces can be balanced if the included angle is 120, so b is correct.

C. When the number of hook codes N 1=2, N3=3, N2=4, the force triangle condition is satisfied, so c is correct.

D. When the hook number N 1= 1, N2=3 and N3=5, a force triangle cannot be formed, so d is wrong.

So choose BC

(2) A. After reaching the balance, it is necessary to mark the position of node O and record the directions of three ropes OA, OB and OC, so A is correct.

B, the length of the rope is meaningless, what is needed is direction, so B is wrong.

C, the hook codes are all the same, and the number of hook codes represents the size of the force, so it is necessary to write down the number of hook codes in each group, so C is correct.

D, hook code is the same, then the number of hook code represents the size of the force, and the balance is not used to measure the quality of hook code, so D is wrong.

So choose AC.

(3)A and Fd are theoretical values, which are the balance forces of Fc, so A is correct.

B, as can be seen from the figure, the resultant force of Fa and Fb is FC.

C, Fe and Fd are the relations between theoretical values and actual values, which are not necessarily equal, so C is wrong.

D, Fe and Fd do not coincide, which means that there is an error between theory and experiment, which does not mean that the experiment failed, so D is wrong.

So choose one

So the answer is: (1) (? b？ C)(2)(？ Answer? c？ )? (3)(? Answer? )