1) 4 - ( 7 - x ) = x - ( 13 - 4
2) a) S1 = 1 + 2 + 3 + ... + 999
b) S2 = 10 + 12 + 14 + ... + 2010
c) S3 = 1 + 4 + 7 + ... + 79
d) S4 = 15 + 17 + 19 + 21 + ... + 151 + 153 + 155
e) S5 = 15 + 25 + 35 + ... + 115
f) S6 = 21 + 23 + 25 + ... + 1001
g) S7 = 24 + 25 + 26 + ... + 125 + 126