T4 Triangle 面积计算公式

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{S_{\triangle ABC}}{\rm{ = }}\left| {{c_1} - {a_1}} \right|\left| {{b_1} - {a_1}} \right| - \frac{1}{2}\left( {\left| {\left( {{b_1} - {a_1}} \right)\left( {{b_2} - {a_2}} \right)} \right| + \left| {\left( {{c_1} - {a_1}} \right)\left( {{c_2} - {a_2}} \right)} \right| + |\left( {{b_1} - {c_1}} \right)\left( {{b_2} - {c_2}} \right)|} \right)