Question

Given the arithmetic sequence with the 3rd tier is 20 and the 7th tribe is 56. the 10th tribe is?

Answer 1

U3 = a + (3-1) b

20 = a + 2b ........... [1]

U7 = a + (7-1) b
56 = a + 6b .......... [2]

Press elimination [1] and [2]:
A + 2b = 20
A + 6b = 56
__________---
-4b = -36
B = 9

Substitution to one of the equations:
20 = a + 2b
20 = a + 2 (9)
20 = a + 18
A = 20-18
A = 2

Enter into the formula:
Un = a + (n-1) b
U10 = 2+ (10-1) 12
U10 = 2+ (9) 9
U10 = 2 + 81
U10 = 83

Answer 2

U10=83.....................