Question

A series is defined by 5+9+13+17+.......+805 a. Find the number of terms of the series. b. Find the sum of the first 20 terms.​

Answer 1

Answer: the number of terms of the series is 201 numbers

              the sum of the first 20 terms is 890    

the number of terms of the series is $\frac{805-5}{4}+1=201$

the series have:

u1 = 5

d = 4

=> u(n) = 5 + 4.(n - 1)

=> u(20) = 5 + 19.4 = 81

so the sum of the first 20 terms is (81 + 5).20/2 = 890

( u1 is the first number of the series, d is the  distance between front and rear numbers, u(n) is the overall term of the sequence, u(20) is the 20th of the series)

Step-by-step explanation: