alexstew1 An=a1 + (n - 1) *d
a15 =-9.1 + 14d
a15 = -9.1 +14 * (-2.5)
a15 = -9.1 + (-35)
a15 = -44.1

ira20151 Sn=(2a1+d(n-1))n/2
S15=(2*-9,1-2,5*14)*15/2
S15=(-18,2-35)*15/2
S15=-53,2*15/2
S15=-399