/**
 * 向量: P=(x2-x1, y2-y1), Q=(x3-x1, y3-y1)
 * 叉积: PQ=(x2-x1)*(y3-y1) - (x3-x1)*(y2-y1)
 *
 * @param line1X
 * @param line1Y
 * @param line2X
 * @param line2Y
 * @param x1
 * @param y1
 * @returns
 */
export declare function crossProduct(line1X: number, line1Y: number, line2X: number, line2Y: number, x1: number, y1: number): number;