A=10 b=20 c=24 найти sn

Question

A=10 b=20 c=24 найти sn

Answer 1

 

1)a1=3*1+5=3+5=8

2)a2=3*2+5=6+5=11

3)d=a2-a1=11-8=3

 

4)a10 = a1+9d=8+3*9=8+27=35

5)a20=a1+19d = 8+3*19=8+57=65

 

6)S10= (a1+a10)/2 *10 = (8+35)*5 = 215

7)S20=(a1+a20)/2 *20 =(8+65)*10 = 73*10=730

8)Sn = n(a1+an)/2