Suppose $v_1,v_1+v_2,v_1+v_2+v_3,...,\sum_{i=1}^nv_i$ are linear independent. Show that $v_1,v_2,...,v_n$ are also linear independent. 
Suppose $v_1,v_1+v_2,v_1+v_2+v_3,...,\sum_{i=1}^nv_i$ are linear independent. Show that $v_1,v_2,...,v_n$ are also linear independent. 

Using the definition of linear independence, $$\alpha_1v_1+\alpha_2(v_1+v_2)+\alpha_3(v_1+v_2+v_3)+...+\alpha_n\sum_{i=1}^nv_i=0,$$ only if $\alpha_1=\alpha_2=...=\alpha_n=0$.
So $\sum_{i=1}^n\alpha_i\sum_{j=1}^iv_j=0$, if I'm correct. 
Therefore $\sum_{i=1}^n\sum_{j=1}^i\alpha_iv_j=0$
I got stuck here and I don't know how to continue.

Let $w_k = v_1+v_2+v_3+\cdots+v_k$. Then
$$
\pmatrix{w_1 \\ w_2 \\ w_3 \\ \vdots \\ w_n}
=
\pmatrix{
1 & 0 & 0 & \cdots & 0 \\
1 & 1 & 0 & \cdots & 0 \\
1 & 1 & 1 & \cdots & 0 \\
&&\cdots\\
1 & 1 & 1 & \cdots & 1 \\
}
\pmatrix{v_1 \\ v_2 \\ v_3 \\ \vdots \\ v_n}
$$
Note that the matrix is non-singular, and so $v_1,\dots, v_n$ can be expressed in terms of $w_1, \dots, w_n$. Therefore, any linear dependence of $w_1,  \dots, w_n$ implies a linear dependence of $v_1, \dots, v_n$.
