Update 13_semiconductors_solutions.md

src/13_semiconductors_solutions.md

@@ -9,11 +9,11 @@ from math import pi
 
 ### Subquestion 1
 
-Electrons near the top of the valence band have a negative effective mass, their energy decreases as $k$ increases from 0, and they have a positive group velocity for $k>0$.
+Electrons near the top of the valence band have a negative effective mass, their energy decreases as $k$ increases from 0, and they have a negative group velocity for $k>0$.
 
 ### Subquestion 2
 
-Holes near the top of the valence band have a positive effective mass, their energy increases as $k$ increases from 0, and they have a positive group velocity for $k>0$.
+Holes near the top of the valence band have a positive effective mass, their energy increases as $k$ increases from 0, and they have a negative group velocity for $k>0$.
 
 ### Subquestion 3