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Previous Year Question (PYQs)



A tree has $2n$ vertices of degree $1$, $3n$ vertices of degree $2$, $n$ vertices of degree $3$. Determine the number of vertices and edges in tree.





Solution

Total number of vertices:

$V=2n+3n+n=6n$

For a tree,

$E=V-1=6n-1$

Sum of degrees:

$2n(1)+3n(2)+n(3)$

$=2n+6n+3n=11n$

By handshaking theorem,

$2E=\text{sum of degrees}$

$2(6n-1)=11n$

$12n-2=11n$

$n=2$

So,

$V=6n=6(2)=12$

$E=V-1=12-1=11$



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