When you are away from the Christmas tree, set the maximum angle of view. Read the picture above and make a CD⊥AB. The bottom of the tree is point E, and the bottom of the person is point F. ..

∴CD=EF=X,AB=a-b,BD=b-c.

In RT△BCD, tan ∠ BCD = BD/CD = b-c/X.

In RT△ACD, tan∠ACD.

=AD/CD=a-c/x .

∴tan∠acb=tan(∠acd-∠abd)=tan∠acd-tan∠bcd/ 1+tan∠acdtan∠bcd

= a-b/[x+(a-c) (b-c)/x] ≤ a-b/2 (a-c) (b-c), if: tana only if x = (a-c) (b-c)/x, that is, x= a-c I hope it will help you. You can ask me if you don't understand.