Question

please help! this is for algebra 2. im not sure that im doing it correctly. thank you!

Answer 1

a)    an = a1 + (n-1)d

so Sn  = (n/2)[ a1 + ( a1 + (n-1)d)]
            = (n/2) [ 2a1 + (n-1)d ]
            =  na1  +  n(n-1) 
                            --------  d
                                2