Triangle 
1⁄2 b*h b=base h=height
b=base h =height

Rectangle 
length * width 
Trapezoid  1⁄2 h ( b_1+ b_2 ) h = height b_1=base 1 b_2=base 2 
Square  s^2 s =side 
Parallelogram  base * altitude 
Pentagon  1⁄2 a*p a = apothem P = perimeter 
Hexagon  1⁄2 a*p a = apothem P = perimeter 
Heptagon  1⁄2 a*p a = apothem P = perimeter 
Octagon 
1⁄2 a*p a = apothem P = perimeter 