Triangle Similarity — AA, SAS, SSS — and Proportional Sides

What is the sum of the interior angles of any triangle?

In a triangle, the angles measure 45° and 65°. What is the measure of the third angle?

Solve for $x$:
$\frac{3}{5} = \frac{x}{20}$

In $\triangle ABC$, $\angle A = 50\degree$ and $\angle B = 70\degree$. In $\triangle DEF$, $\angle D = 50\degree$ and $\angle E = 70\degree$. Are the triangles similar? If so, by which criterion?

Triangle 1 has sides 4, 6, and 8. Triangle 2 has sides 6, 9, and 12. Are the triangles similar?

$\triangle ABC \sim \triangle DEF$. If $AB = 10$, $DE = 6$, and $BC = 15$, find $EF$.

$\triangle PQR \sim \triangle XYZ$. If $PQ = 8$, $QR = 12$, $XY = 6$, find $YZ$.

In $\triangle ABC$, $AB = 12$, $AC = 18$, and $\angle A = 55\degree$. In $\triangle DEF$, $DE = 8$, $DF = 12$, and $\angle D = 55\degree$. Are the triangles similar?

$\triangle ABC \sim \triangle DEF$ with scale factor $k = 3$ (where $k = AB/DE$). If $AC = 21$, find $DF$.