RAJASTHAN ­ PET Rajasthan PET Solved Paper-2007

  • question_answer
    The value of\[^{50}{{C}_{4}}+\sum\limits_{r=1}^{6}{^{56-r}{{C}_{3}}}\]is

    A)  \[^{55}{{C}_{4}}\]                

    B)  \[^{55}{{C}_{3}}\]

    C)  \[^{56}{{C}_{3}}\]                

    D)  \[^{56}{{C}_{4}}\]

    Correct Answer: D

    Solution :

     \[^{50}{{C}_{4}}{{+}^{55}}{{C}_{3}}{{+}^{54}}{{C}_{3}}{{+}^{53}}{{C}_{3}}{{+}^{52}}{{C}_{3}}\] \[{{+}^{51}}{{C}_{3}}{{+}^{50}}{{C}_{3}}\] \[{{=}^{50}}{{C}_{3}}{{+}^{50}}{{C}_{4}}{{+}^{51}}{{C}_{3}}{{+}^{52}}{{C}_{3}}{{+}^{53}}{{C}_{3}}{{+}^{54}}{{C}_{3}}\] \[{{+}^{55}}{{C}_{3}}\] \[{{=}^{51}}{{C}_{4}}{{+}^{51}}{{C}_{3}}{{+}^{52}}{{C}_{3}}{{+}^{53}}{{C}_{3}}{{+}^{54}}{{C}_{3}}{{+}^{55}}{{C}_{3}}\] \[{{=}^{52}}{{C}_{4}}{{+}^{52}}{{C}_{3}}{{+}^{53}}{{C}_{3}}{{+}^{54}}{{C}_{3}}{{+}^{55}}{{C}_{3}}\] \[{{=}^{53}}{{C}_{4}}{{+}^{53}}{{C}_{3}}{{+}^{54}}{{C}_{3}}{{+}^{55}}{{C}_{3}}\] \[{{=}^{54}}{{C}_{4}}{{+}^{54}}{{C}_{3}}{{+}^{55}}{{C}_{3}}\] \[{{=}^{55}}{{C}_{4}}{{+}^{55}}{{C}_{3}}\] \[{{=}^{56}}{{C}_{4}}\]


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