Question

3. If an= n2+4 find ano and ag and find the sum of first 20 terms​

Answer 1

Answer:

$\large S_{20}=670$

Step-by-step explanation:

Given :

$\large a_n=n^2+4\\\\\\\\\large \text{Put n = 1}\\\\\\\large a_1=1+4\implies5\\\\\\\large \text{Put n = 2}\\\\\\\large a_2=4+4\implies8\\\\\\\large \text{We get first term a = 5}\\\\\\\large \text{common difference ( d ) =8-5\implies3 }$

Now we have n = 20 , d = 3 and a = 5

We know sum formula

$\large S_n=\dfrac{n}{2}[2a+(n-1)d]\\\\\\\large S_{20}=\dfrac{20}{2}[2\times5+(20-1)3]\\\\\\\large S_{20}=10[2\times5+19\times3]\\\\\\\large S_{20}=10[10+57]\\\\\\\large S_{20}=10[67]\\\\\\\large S_{20}=670$

Thus we get answer.