Given: A tree casts a 9 ft shadow at the same time that a person 6 ft tall casts a 4 ft shadow.
Since both the tree and the man are standing vertical to the ground at the same time so the sun rays will make the same angle , therefore, the triangles made by both tree and man are similar.
And in the similar triangles, the sides are proportional.
You need to measure the angles wich means that u use a protractor if the angle is 90 degrees is a straight angle if it is below 90 degrees is acute and if it is above 90 degrees it's obtuse