Which of the following choices is the common difference between the terms of this arithmetic sequence?

3x + 7y, 8x + 4y, 13x + y, 18x - 2y, 23x - 5y,

a. -5x - 3y
b. 3x + 7y
c. 5x - 3y
d. 5x + 3y
e. 3x - 7y

List x1, x2, x3, x4 where xi is the left endpoint of the four equal intervals used to estimate the area under the curve of f(x) between x = 4 and x = 6. a 4, 4.5, 5, 5.5 b 4.5, 5, 5.5, 6 c 4.25, 4.75, 5.25, 5.75 d 4, 4.2, 5.4, 6