MCQ
$4 .×^{ 15}P_r = 3. 16P_{r-1}$ હોય, તો $r = ...........$
- A$10$
- B$11$
- C$12$
- D$13$
$4.\frac{{15!}}{{(15 - r)!}} = 3.\,\frac{{16!}}{{(17 - r)!}}\,$
$4.\frac{{15!}}{{(15 - r)!}} = 3.\frac{{16 \times 15!}}{{(17 - r)(16 - r)(15 - r)!}}$
$4 = \frac{{48}}{{(17 - r)(16 - r)}}\,\,$
$(17 - r)(16 - r) = 4 \times 3\, = \,(17 - 13)(16 - 13)$
$\,r\,\, = \,\,13$ મળે
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