Step-by-step explanation:
12. n^2+2n
if you insert 1 for k and then work up by inserting 2 for k and adding those together and stoping at n.
13. 8-2(2^n)
if you insert 3 for k and then work up by inserting 4 for k and adding those together and keep on going but stopping at n.
Hope that helps :)
