JEE Main & Advanced Sample Paper JEE Main Sample Paper-22

  • question_answer
    Let \[a={{i}^{{{k}_{1}}}}+{{i}^{{{k}_{2}}}}+{{i}^{{{k}_{3}}}}+{{i}^{{{k}_{4}}}},(i=\sqrt{-1})\] where each \[{{k}_{a}}\] is randomly chosen from the set {1, 2, 3,4}. The probability that a = 0, is

    A)  \[\frac{7}{64}\]                                    

    B)   \[\frac{9}{64}\]

    C)  \[\frac{37}{256}\]                    

    D)  \[\frac{39}{256}\]

    Correct Answer: B

    Solution :

    The powers of i contain two sets of numbers that are additive inverses of each other, namely (1, - 1) and (i, -i). Thus the only sets of four numbers that will satisfy a = 0 are permutations of either (1, 1, -1, -1),  and (i, -i 1-1). The first two have \[^{4}{{C}_{2}}=6\]  distinct arrangement each, while the has 4! = 24 total arrangement, giving 2(6) + 24 = 36 overall. There are \[{{4}^{4}}=256\] possibilities, giving a probability of \[\frac{36}{256}\,=\frac{9}{64}\].


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