for part (a) find the sum of the first n terms of the arithmetic series. for part (b) find n for the given sum Sn.
45. 3+8+13+18+23+...
a. n=20
b. Sn=366
46. 50+42+34+26+18+...
a. n=40
b. Sn=182
47. -10+(-5)+0+5+10+...
a. n=19
b. Sn=375
48. 34+31+28+25+22+...
a. n=32
b. Sn=-12
49. 2+9+16+23+30+...
a. n=68
b. Sn=1661
50. 2+16+30+44+58+...
a. n=24
b. Sn=2178
