The 4th term of an AP is 6 if the um of the 8th and 9th term is -72, The 4th term of an AP is -18.
The sum of 8th and 9th term of an AP is given as -72.
We can use the formula for the sum of any n terms of an AP,
Sn = n/2[2a + (n-1)d]
Where,
a = first term
d = common difference
n = number of terms
Therefore,
-72 = 8/2[2a + (8-1)d]
-72 = 4[2a + 7d]
-18 = 2a + 7d
Subtracting 7d from both the sides,
-18 - 7d = 2a
Dividing both the sides by 2,
(-18 - 7d)/2 = a
a = -18/2 - (7d)/2
a = -9 - (7d)/2
For the 4th term of an AP,
a4 = a + (4-1)d
=-9 - (7d)/2 + 3d
=-9 + 6d
=-9
Learn more about AP here
https://brainly.com/question/27593513
#SPJ4
