S21=(2a1+20d)*n/2
a1=3
a2=a1+d a1+d=-3 3+d=-3 d=-6
{a1=3
{d=-6
S21=(2*3+20*(-6))*21/2
S21=(6-120)*21/2=-114*21/2=-57*21=-1197
S21= -1197
S21=(2a1+20d)*n/2
a1=3
a2=a1+d a1+d=-3 3+d=-3 d=-6
{a1=3
{d=-6
S21=(2*3+20*(-6))*21/2
S21=(6-120)*21/2=-114*21/2=-57*21=-1197
S21= -1197