Plane Geometry
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Back to topTriangle

Perimeter, P=a+b+c
Semi-perimeter, s=12P=12(a+b+c)
Sum of included angles, A+B+C=180∘
Area, A=12bh=12absinθ=√s(s−a)(s−b)(s−c)
Quadrilateral

Perimeter, P=a+b+c+d
Semi-perimeter, s=12P=12(a+b+c+d)
Sum of included angles, A+B+C+D=360∘

Area, A=√(s−a)(s−b)(s−c)(s−d)−abcdcos2φ=12d1 d2sinθ
where, φ=12(A+C) or φ=12(B+D)
Regular Polygons

Perimeter, P=nx
Central angle of one segment, θ=360∘n
Area, A=nA1=n2xr=n2R2sinθ
Circle

Circumference, c=2πr=πd
Area, A=πr2=π4d2