a)

a13	=	1+(13-1)*1)	=	13
a18	=	1+(18-1)*1)	=	18
s10	=	10(2*1+9*1)          2	=	55
s20	=	20(2*1+19*1)        2	=	210

b)

a8	=	3+(8-1)*2)	=	17
a11	=	3+(11-1)*2)	=	23
s8	=	8(2*3+7*2)          2	=	80
s14	=	14(2*3+13*2)        2	=	224

c)

a6	=	2+(6-1)*3)	=	17
a12	=	2+(12-1)*3)	=	35
a18	=	2+(18-1)*3)	=	53
s10	=	10(2*2+9*3)        2	=	155

d)

a17	=	4+(17-1)*5)	=	84
a24	=	4+(24-1)*5)	=	119
s6	=	6(2*4+5*5)       2	=	99
s24	=	24(2*4+23*5)        2	=	1476