a1=3.1, d = 0.9
S9 = 9(a1+a9)/2 = 9(a1+a1+8d)/2 = 9(2a1+8d)/2 = 9(a1+4d) = 9(3.1+4•0.9) = 9(3.1+3.6) = 9•6.7 = 60.3
a1=3.1, d = 0.9
S9 = 9(a1+a9)/2 = 9(a1+a1+8d)/2 = 9(2a1+8d)/2 = 9(a1+4d) = 9(3.1+4•0.9) = 9(3.1+3.6) = 9•6.7 = 60.3