BV1kS4y1Y7EB

有

cos∠BAC=-1/9

即

BC

=√(32+12-2·3·1·(-1/9)）

=4√6/3

且

AD為角分線

即

BD/CD

=AB/AC

=3

即

BD=√6

CD=√6/3

即

AD

=√(AB·AC-BD·CD)

=√(3·1-√6·√6/3)

=1

BV1kS4y1Y7EB

有

cos∠BAC=-1/9

即

BC

=√(32+12-2·3·1·(-1/9)）

=4√6/3

且

AD為角分線

即

BD/CD

=AB/AC

=3

即

BD=√6

CD=√6/3

即

AD

=√(AB·AC-BD·CD)

=√(3·1-√6·√6/3)

=1