Let the extremities of the double ordinate be P(pt², 2pt); p'(pt², -2pt).
Length of double ordinate = pp' = 4pt = 8p$\Rightarrow$t = 2
Then P = (4p, 4p); p' = (4p, -4p)
Slope of op = 1; slope of op' = -1
m₁m₂ = -1$\Rightarrow$$\mathrm{\theta }$ = 90^o