Answer:
The 7th term is 10 and the 9th term is 16
(and the common difference d = 3)
Explanation:
If by calculate the numbers, you mean the 7th term and 9th term, first, you will determine the common difference.
The nth term of an A.P is given by the formula
Tₙ= a+(n-1)d
Where Tₙ is the nth term
a is the first term
and d is the common difference
From the question,
a = -8
T₇ : T₉ = 5:8
From the formula
T₇ = a + (7-1)d = a + 6d
and T₉ = a + (9-1)d = a + 8d
Then.
a + 6d : a + 8d = 5:8
But a = -8
∴ -8 + 6d : -8 + 8d = 5:8
We can write that
(-8 + 6d) / (-8 + 8d) = 5/8
Cross multiply
8(-8+6d) = 5(-8+8d)
-64 + 48d = -40 + 40d
48d - 40d = -40 + 64
8d = 24
d = 24/8
d = 3
∴ The common difference is 3
Now, for the 7th term
From
T₇ = a + 6d
T₇ = -8 + 6(3)
T₇ = -8 + 18
T₇ = 10
and for the 9th term
T₉ = a + 8d
T₉ = -8 + 8(3)
T₉ = -8 + 24
T₉ = 16
Hence, the 7th term is 10 and the 9th term is 16