Assume that Q, R, and S are the angles of a triangle, with opposite sides q, r, and s respectively. Select all of the following that represent the law of cosines.

The law of cosines says that the square of a side equals the sum of the squares of the other two sides minus twice their product times the cosine of the angle between them. Thus q² = r² + s² − 2rs cos(Q), and similarly r² = q² + s² − 2qs cos(R) and s² = q² + r² − 2qr cos(S). These match A, D, and E; B uses the wrong side lengths with angle R, and C uses the wrong angle for the equation for r².

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