两个人轮流抛硬币，正面获胜，先抛的人获胜的概率_两个人扔硬币，扔到正面的赢，先扔赢的概率

A先，B后

P(A) = 1/2 +      //A直接取胜 
	   1/2 * 1/2 * 1/2 +      // A1失败B1失败A2取胜
	   1/2 * 1/2 * 1/2 * 1/2 *1/2  +    //A1失败B1失败A2失败B2失败A3取胜
	   ...

p = 1/2 + (1/2)^3 + (1/2)^5 + (1/2)^7 + ...
等比数列求和
p = 1/2 * (1 - (1/4)^n) / (1 - 1/4) = 2/3




P(B） = 1/2 * 1/2 +     // A1失败B1取胜
        1/2 * 1/2 * 1/2 * 1/2 +     // A1失败B1失败A2失败B2取胜
        1/2 * 1/2 * 1/2 * 1/2 * 1/2 * 1/2 +  //A1失败B1失败A2失败B2失败A3失败B3取胜、
        ...    
p = (1/2)^2 + (1/2)^4 + (1/2)^6 + ..

p = 1/4 * (1 - (1/4)^n) / (1 - 1/4)
		
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