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Question Number 40216 by scientist last updated on 17/Jul/18
Answered by math1967 last updated on 17/Jul/18
$$\overset{{n}} {\:}{c}_{{r}} +\overset{{n}} {\:}{c}_{{r}−\mathrm{1}} +\:^{{n}} {c}_{{r}−\mathrm{1}} +\overset{{n}} {\:}{c}_{{r}−\mathrm{2}} \\ $$$$\overset{{n}+\mathrm{1}} {{c}}_{{r}} +\overset{{n}+\mathrm{1}} {\:}{c}_{{r}−\mathrm{1}} =\overset{{n}+\mathrm{2}} {\:}{c}_{{r}} \\ $$