Question

Please Help I Don't Undertand.

Answer 1

If triangles AMN and ABC are similar, then

AM/AB = AN/AC

or

AM/(AM + MB) = AN/(AN + NC)

Check if this is true:

AM/AB = 21/(21 + 9) = 21/30 = 7/10

AN/AC = 14/(14 + 6) = 14/20 = 7/10

The angle at vertex A is common to both of the triangles.

Then by the side-angle-side (SAS) similarity theorem, the triangles are indeed similar.