Firstly, the height of the tree corresponding to the shadow length falling on the ground is calculated according to the ratio of the height of the object to the shadow length at the same moment, and the shadow length falling on the wall is the height of the tree.

Maybe you can understand these two pictures better.

Solution: Because the height of the object is also proportional to the length of the shadow,

So:

Height of measuring pole: the shadow length of measuring pole = the tree height corresponding to the shadow length falling on the ground: the shadow length falling on the ground.

Namely: 1: 0.9 = the height of the tree corresponding to the shadow length falling on the ground: 2.7.

Solution: The height of the tree corresponding to the shadow length falling on the ground =3m,

So the height of the tree is: 3+1.2 = 4.2m. 。