JAMB - Mathematics (1995 - No. 16)
The 4th term of an A.P. is 13 while the 10th term is 31. Find the 21st term.
175
85
64
45
คำอธิบาย
a + 3d = 13
a + 9d = 31
6d = 18
= d = 3
a = 13 - 9
= 4
a + 20d = 4 + (20 x 3)
= 64
a + 9d = 31
6d = 18
= d = 3
a = 13 - 9
= 4
a + 20d = 4 + (20 x 3)
= 64