Question 998451
1	a	+	4	d	=	13	.............1	
Total value								
1	a	+	14	d	=	17	.............2	
Eliminate	d							
multiply (1)by		-7						
Multiply (2) by		2						
-7	a		-28	d	=	-91		
2	a	+	28	d	=	34		
Add the two equations								
-5	a				=	-57		
/	-5							
a	=	11						
plug value of			a	in (1)				
1	a	+	4	d	=	13		
11		+	4	d	=	13		
			4	d	=	13		-11
			4	d	=	2		
				d	=	1/2
		

a= 11, d= 1/2 , n=15

Find {{{S[n] }}}

{{{S[n] = (n/2)(2a+(n-1)d))}}}


Plug in the values to get {{{S[15]}}}= 217.5