Question

Find the arithmetic sequence whose fifth term = 21, and the tenth term = 3 times the second term

Answer 1

by our definitions:
a + 4d = 21 ---> a = 21-4d **

a+ 9d = 3(a+d)
a + 9d = 3a + 3d
2a = 6d
a = 3d ***

equate ** and ***
3d = 21-4d
7d = 21

carry on