APL, 37 (28)

Upright triangle (37-char)

({(x,⍵,x←' '⍴⍨2⍴1⌷⍴⍵)⍪⍵,⍵}⍣⎕)1 2⍴'/\' 

• 1 2⍴'/\': Create a 1×2 character matrix /\
• {(x,⍵,x←' '⍴⍨2⍴1⌷⍴⍵)⍪⍵,⍵}: A function that pad the right argument on both sides with blanks to create a matrix double as wide, then laminates the right argument itself doubled onto the bottom.
  E.g. /\ would become

Skewed Triangle (28-char)

({(⍵,' '⍴⍨⍴⍵)⍪⍵,⍵}⍣⎕)1 1⍴'○' 

• 1 1⍴'○': Create a 1×1 character matrix ○
• {(⍵,' '⍴⍨⍴⍵)⍪⍵,⍵}: A function that pad the right argument on the right with blanks to create a matrix double as wide, then laminates the right argument itself doubled onto the bottom.
  E.g. ○ would become

APL, 37 32 (28 23)

Upright triangle (37 32-char)

({((-1⌷⍴⍵)⌽⍵,∊⍵)⍪⍵,⍵}⍣⎕)1 2⍴'/\' 

• 1 2⍴'/\': Create a 1×2 character matrix /\
• {((-1⌷⍴⍵)⌽⍵,∊⍵)⍪⍵,⍵}: A function that pad the right argument on both sides with blanks to create a matrix double as wide, then laminates the right argument itself doubled onto the bottom.
  E.g. /\ would become

Skewed Triangle (28 23-char)

({(⍵,∊⍵)⍪⍵,⍵}⍣⎕)1 1⍴'○' 

• 1 1⍴'○': Create a 1×1 character matrix ○
• {(⍵,∊⍵)⍪⍵,⍵}: A function that pad the right argument on the right with blanks to create a matrix double as wide, then laminates the right argument itself doubled onto the bottom.
  E.g. ○ would become

• 1 2⍴'/\': Create a 1×2 character matrix /\
• {(x,⍵,x←' '⍴⍨2⍴1⌷⍴⍵)⍪⍵,⍵}: A function that pad the left argument on both sides with blanks to create a matrix double as wide, then laminates the left argument itself doubled onto the bottom.
  E.g. /\ would become

• 1 1⍴'○': Create a 1×1 character matrix ○
• {(⍵,' '⍴⍨⍴⍵)⍪⍵,⍵}: A function that pad the left argument on the right with blanks to create a matrix double as wide, then laminates the left argument itself doubled onto the bottom.
  E.g. ○ would become

• 1 2⍴'/\': Create a 1×2 character matrix /\
• {(x,⍵,x←' '⍴⍨2⍴1⌷⍴⍵)⍪⍵,⍵}: A function that pad the right argument on both sides with blanks to create a matrix double as wide, then laminates the right argument itself doubled onto the bottom.
  E.g. /\ would become

• 1 1⍴'○': Create a 1×1 character matrix ○
• {(⍵,' '⍴⍨⍴⍵)⍪⍵,⍵}: A function that pad the right argument on the right with blanks to create a matrix double as wide, then laminates the right argument itself doubled onto the bottom.
  E.g. ○ would become