Proving $(n+1)^2+(n+2)^2+...+(2n)^2= \frac{n(2n+1)(7n+1)}{6}$ by induction My question: $(n+1)^2+(n+2)^2+(n+3)^2+...+(2n)^2= \frac{n(2n+1)(7n+1)}{6}$
My workings


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*LHS=$2^2$ =$4$ RHS= $\frac{24}{6} =4 $

*$(k+1)^2+(k+2)^2+(k+3)^2+...+(2k)^2= \frac{k(2k+1)(7k+1)}{6}$

*LHS (subsituting $n= k+1$)----> $(k+2)^2+(k+3)^2+(k+4)^2+...+(2k)^2+(2k+1)^2$
Now, this is where my problem has started. When I substitute $n=k+1$ in the third step, I do not have the $(k+1)^2$ anymore. So I cannot use the statement in second step. So my question is what should i do next? 

This is 2nd tough-degree exercise in induction. Observe that both the first and last elements change, so if $\;k=n\;$ we get
$$(n+1)^2+(n+2)^2+\ldots+(2n)^2=\frac{n(2n+1)(7n+1)}6\;\;(**)$$
then for $\;k=n+1\;$ we get
$$(n+1+1)^2+(n+3)^2+\ldots+(2n)^2+(2n+1)^2+\overbrace{(2n+2)^2}^{=(2(n+1))^2}=\frac{(n+1)(2n+3)(7n+8)}6$$
Now, we prove the last one assuming the first one. Developing left side:
$$(n+2)^2+\ldots+(2n)^2+(2n+1)^2+(2n+2)^2=(n+1)^2+\ldots+(2n+2)^2-(n+1)^2=$$
$$=(n+1)^2+\ldots+(2n)^2+(2n+1)^2+(2n+2)^2-(n+1)^2\stackrel{\text{Ind. Hyp.}}=$$
$$=\frac{n(2n+1)(7n+1)}6+(2n+1)^2+4(n+1)^2-(n+1)^2=$$
$$=\frac{n(2n+1)(7n+1)}6+(2n+1)^2+3(n+1)^2=\frac{(2n+1)\left[n(7n+1)+12n+6\right]+18(n+1)^2}6=$$
$$=\frac{(2n+1)(7n^2+13n+6)+18n^2+36n+18}6=\frac{(n+1)(2n+3)(7n+8)}6$$
The last equality you can prove it opening parentheses and equating coefficients, or showing the left side has as roots $\;-1,\,-3/2,\,-8/7\;$
Alternative way to prove:
$$(n+1)^2+(n+2)^2+\ldots+(n+n)^2=$$
$$=n^2+\ldots+n^2+2n+4n+\ldots+2nn+1+2^2+\ldots+n^2=$$
$$n\cdot n^2+2(1+2+\ldots+n)n+\frac{n(n+1)(2n+1)}6=$$
$$=n^3+n^2(n+1)+\frac{n(n+1)(2n+1)}6=\ldots$$
