Which term of the AP 80, 76, 72, 68……. is the first negative term?

Here a = 80 and d = 76 – 80 = – 4
Let T_n be the first negative term of the given AP.
T_n < 0
a + (n – 1)d < 0
80 + (n – 1) – 4 < 0
80 – 4n + 4 < 0
84 < 4n
n > 21
Hence the 22^th term of the given AP will be its first negative term.