JEE Advance - Mathematics (2012 - Paper 2 Offline - No. 1)
Explanation
Given, $$b_n$$ denotes the number of $$n$$-digit integer formed by the digits 0, 1 or both such that $$n$$-digit integer ending with 1 and no consecutive digits are '0'.
$$\therefore \quad b_6=$$ six digit number ending with 1.
Like 1 ........... 1, and rest four places are filled as Case No. (I) : Use four ' 1 '
$$\text { Case No. (I) : Use four ' } 1 \text { ' }$$
$$\underline 1 \underbrace {\underline 1 \,\underline 1 \,\underline 1 \,\underline 1 }_{}\underline 1 $$
Number of ways = 1
Case No. (II) : Use three '1' and one '0'
$$\underline 1 \underbrace {\underline 1 \,\underline 1 \,\underline 1 \,\underline 0 }_{}\underline 1 $$
Number of ways $$=\frac{4!}{3!}=4$$
Case No. (III) : Use two '1' and two '0'
$$\underline 1 \underbrace {\underline 1 \,\underline 0 \,\underline 1 \,\underline 0 }_{}\underline 1 $$
or
$$\underline 1 \underbrace {\underline 1 \,\underline 1 \,\underline 0 \,\underline 1 }_{}\underline 1 $$
or
$$\underline 1 \underbrace {\underline 0 \,\underline 1 \,\underline 1 \,\underline 0 }_{}\underline 1 $$
No. of ways = 3
Hence, $$b_6=1+4+3=8$$
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