Work out: $${{3}^{2}}+{{6}^{2}}+{{9}^{2}}+\cdots +{{30}^{2}}$$.  Pip\textquotesingle s answer:  $$=3\times ({{1}^{2}}+{{2}^{2}}+{{3}^{2}}+\cdots +{{10}^{2}})$$  $$=3\times 10\times(10+1)\times(2\times 10+1)\div 6$$  $$=1155$$ 
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