(AG*AIsin∠IAG)^2=(lAGllAIl)^2(sin∠IAG)^2
=(lAGllAIl)^2{1-(cos∠IAG)^2}
=(lAGllAIl)^2-(lAGllAIl(cos∠IAG))^2
=(lAGllAIl)^2-(AG*AI)^2

AG*AIはベクトルAG、AIの内積