If the coefficient of (2r + 4)th term is equal to the coefficient of (r – 2)th term in the expansion of (1+x)18 then r =
1
2
3
4
Coefficient of (2r + 1)th term = Coefficient of (4r + 5)th term ⇒10C2r=10C4r+4⇒2r+4r+4=10⇒6r=6=⇒1