A1=1, an = 4a n-1 - 1

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