1) a1=1, an=133, d=2
an=a1+(n-1)*d
133=1+(n-1)*2
132=2n-2, n=67
S67=((1+133)/2)*67=67*67=4489
2) a1=10, d=11-10=1, an=99
99=10+(n-1)
n=90
S90=((10+99)/2)*90=109*45=4905
a1=100, d=101-100=1, an=999
an=a1(n-1)*d
999=100+(n-1)
n=900
S900=((a1+a900)/2)*900
S900=((100+999)/2)*900=1099*450=494550
1) a1=1, an=133, d=2
an=a1+(n-1)*d
133=1+(n-1)*2
132=2n-2, n=67
S67=((1+133)/2)*67=67*67=4489
2) a1=10, d=11-10=1, an=99
an=a1+(n-1)*d
99=10+(n-1)
n=90
S90=((10+99)/2)*90=109*45=4905
a1=100, d=101-100=1, an=999
an=a1(n-1)*d
999=100+(n-1)
n=900
S900=((a1+a900)/2)*900
S900=((100+999)/2)*900=1099*450=494550