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ΔABC

∠9 = ∠19 + ∠21

∠7 = ∠20 + ∠21

∠8 = ∠19 + ∠20

ΔXYZ

∠3 = ∠11 + ∠12

∠5 = ∠10 + ∠12

∠4 = ∠10 + ∠11

ΔNOR

∠1 = ∠5 + ∠6

∠2 = ∠ 4 + ∠5

∠3 = ∠4 + ∠6

Step-by-step explanation:

According to the exterior angle theorem, the exterior angle of a triangle is equal in measurement to the sum of the two interior angles of the triangle;

In triangle ΔABC, we have;

∠9 = ∠19 + ∠21

∠7 = ∠20 + ∠21

∠8 = ∠19 + ∠20

In triangle ΔXYZ, we have;

∠3 = ∠11 + ∠12

∠5 = ∠10 + ∠12

∠4 = ∠10 + ∠11

In triangle ΔNOR, we have;

∠1 = ∠5 + ∠6

∠2 = ∠ 4 + ∠5

∠3 = ∠4 + ∠6