At first glance, they are brothers who appear falsely similar.

Solving a linear equation is difficult.

Solving a quadratic equation is difficult.

He worked all night to find the solution to the partial differential equation.

I am learning how to solve ordinary differential equations.

That plan is too impractical, it's difficult to implement.

His plan was so unrealistic that no one could believe it.

He works at a non-governmental organization.

His glasses are for correcting astigmatism.

This design has a unique charm because it is asymmetrical.