Answer :
Arithmetic sequence is modeled by the next formula:
[tex]a_n=a_1+(n-1)\cdot d[/tex]where an is the nth term, a1 is the first term, and d is the common difference.
In the case of the first sequence:
[tex]a_n=1+(n-1)\cdot4[/tex]In the case of the second sequence:
[tex]a_n=1+(n-1)\cdot5[/tex]The 6th term of the first sequence is:
[tex]a_6=1+(6-1)\cdot4=21[/tex]The 5th term of the second sequence is:
[tex]a_5=1+(5-1)\cdot5=21[/tex]The 11th term of the first sequence is:
[tex]a_{11}=1+(11-1)\cdot4=41[/tex]The 9th term of the second sequence is:
[tex]a_9=1+(9-1)\cdot5=41[/tex]Therefore, the first three common terms are: 1, 21, and 41. And the sum of them is 63