Sector area formula: A = ½r²θ
A sector with central angle θ on a circle of radius r has area ½r²θ, with θ in radians. It is the whole circle's πr² scaled by the fraction θ/2π that the sector occupies. Pair it with the arc-length formula for any "slice of a circle" question.
circlesector
Related Basics identities
Practise it
Reading a formula is not the same as recalling it under time pressure. The workspace drills Basics questions in the quiz, and the triangle solver shows a worked solution for any triangle you type in.
Or browse all 45 identities in the full reference, or read the trigonometry guides for the method behind them.