Answer
True
Work Step by Step
We have $T:R^8 \rightarrow R^3$ is onto, then $Rng(T)=R^3\\
\rightarrow \dim [Rng(T)]=3$
Apply Rank Nullity Theorem:
$\dim [Ker(T)]+\dim [Rng(T)]=\dim R^8\\
\dim [Ker (T)]=5$
Hence, the statement is true.
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