MAHARASHTRA Class 10: Tests of Similarity of Triangles Videos | Revision on concepts related to similar triangles

Thale’s theorem states that if a line is drawn parallel to one side of a triangle intersecting the other two sides, then it divides the two sides in the same ratio. 

Similar triangles have their corresponding sides in equal ratios and corresponding angles equal. 

In ∆ABC, DE is parallel to BC.

So by BPT,

AD/DB = AE/EC

(x + 3)(x − 3) = x(x − 1)

x² − 9 = x² - x

x = 9