Question

a1=12 d=-2,5 a10-? s3-?

Answer 1

a10=a1+9d=12-22,5=-10,5 a3=a1+2d=12-5=7 S3=3*(a1+a3)/2=28,5

Answer 2

а10=а1+d(n-1)=12-2,5(10-1)=12-22,5=-10,5
S3=(2a1+d(n-1))*n/2=(2*12-2,5*2)*3/2=57/2=28,5