Question 18
Find all functions f from the reals to the reals such that
$$ \left(f\left(x\right)+f\left(z\right)\right)\left(f\left(y\right)+f\left(t\right)\right)=f\left(x y-z t\right)+f\left(x t+y z\right) $$
for all real x, y, z, t.
$$ \left(f\left(x\right)+f\left(z\right)\right)\left(f\left(y\right)+f\left(t\right)\right)=f\left(x y-z t\right)+f\left(x t+y z\right) $$
for all real x, y, z, t.