Triangle
P = a+b+c
A = (b*h)/2
Right Triangle
P = a+b+c
A = (a*b)/2
Also, remember: a2+b2 = c2
Equilateral Triangle
Rectangles
A=a*b, P = 2(a+b)
The square is a special rectangle whose sides are all equal (a=b). Thus for a square:
A = a2, P = 4a
Rhombus
A = (d1*d2)/2, also A = a*h
P = 4a
Parallelogram
A = b*h, P = 2(a+b)
Trapezoids
A = (a+b)*h/2, P = a+b+c+d
Circle
A = π*r2, P = 2πr
A=(r2*θ)/2 , P = r(θ+2)
For the annulus aka the ring of the circle you need the main radius of the circle (r) and the smaller radius of the new ring(r'). Thus:
A = π(r2-r'2)
For any circle segment:
(r2/2) *(θ - sinθ)