Use of Let?

Identify the segment of sentence which have gramatical error: :Let us think how can we solve the problem amiably.

Let us consider the Example 3 of Section 3.2. Will the rails cross each other?

Let us check the dimensional correctness of the relation v = u + at .

If (sin theta+cos theta)/(sin theta-cos theta)=7, then let us write by calculating,the value of tan theta