solution

Judging from the meaning of the question.

English generally doesn't say that. It's too lax. Specifically, what conditions can be obtained: from ... we have ...

Because so

Because ... therefore. ...

Let AB be parallel to DC

Draw AB line parallel to DC

Let EF be perpendicular to GH and F.

Draw EF perpendicular to GH at F.

finally

In summer, ...

Get a solution

Solve it, and we have it. ...

The only possibility is that

If and only if. The abbreviation is iff.