The line lx + my + n = 0 will touch tho parabola y ² = 4ax, if ________
(a) mn = al ²             (b) lm = an ²             (c) ln = am ²                (d) (a) mn = al

Dear student

We can write lx + my + n = 0 in the form of y = Mx + c

So, my = -lx - n
y = -lx/m - n/m

Comparing with y = Mx + c,
M = -l/m
c = -n/m

We know that c = a/M
Mc = a

Putting the values of M and c,
-l/m x - n/m = a
ln/m^2 = a
ln = am^2


Regards