-10 is the second term.
Step-by-step explanation:
After the first term each term is obtained from the previous one by adding 10.
c(1) = -20
so c(2) = -20 + 10
= -10.
I can use variables to represent the coordinates of the vertices for a general triangle, ∆ABC. Then I can calculate the midpoints of the sides in terms of those variables. Using the point-slope formula for the equation of a straight line, I can build the symbolic equations for the three medians, AE, BF, and CD. I can solve for the point of intersection for two of the medians, AE and BF, for example. Finally, I can prove the lines (i.e., medians) concurrent if the point I found also satisfies the equation of the line for CD.