The series $-2+10-50+250$ can be written using sigma notation (also called summation notation): > $\large\displaystyle\sum\limits_{k=0}^{m}{{a_k}}$ **Find $m$.** $m=$ [[☃ numeric-input 2]] **Write an expression for $a_k$ in terms of $k$.** $a_k=$ [[☃ expression 1]]