The missing terms in AP: __, 13, __, 3 are:

a₂ = 13 and
a₄ = 3
The nth term of an AP;
a_n = a+(n−1) d
a₂ = a +(2-1)d
13 = a+d ………………. (i)
a₄ = a+(4-1)d
3 = a+3d ………….. (ii)
Subtracting equation (i) from (ii), we get,
– 10 = 2d
d = – 5
Now put value of d in equation 1
13 = a+(-5)
a = 18 (first term)
a₃ = 18+(3-1)(-5)
= 18+2(-5) = 18-10 = 8 (third term)