2.c, simplify the original formula: (1/3-i) (1-2i)/[(1+2i) (1-2i)] =-2/15-i/.

3, c, because the frequency greater than 125 is 0.002*20=0.04, and the corresponding number of days =0.04*200=8 days.

4, B, because A equals 2 and B is different, then C is different and E is different.

5, b, because when n=0, s =1; When n= 1, s = 4；; When n=2, s =13; When n=3, s=38 and n=4, it jumps out of the loop.

6, d, I can't see clearly what power it is. According to the answer, it should be 6 power, so C (6 6,2) * 24/(-3) 2 = 80/3.

7, c, because a(n- 1)=a(n)/q, a(n+ 1)=a(n)*q, it can be solved by substituting it into the original equation. Note: n in a(n) stands for subscript.

Son, these are basic questions. Looks like you're coming to see us, hehe.

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