So AD? +BD？ =AB？ = 100

So BDA = 90.

So AD is the middle vertical line of BC, so AC=AB= 10.

2.① if c = 90, then c = b/cos 30 = 20/√ 3 = 20 √ 3/3, and a = csin30 =1/2c =10 √ 3/3.

If b = 90, then a = BSIN 30 = 5, and c = BCOS 30 = 5 √ 3.

② if c = 90, then c = a/sin60 = 2 √ 6/√ 3 = 2 √ 2, and b = ccos60 = 1/2c = √ 2.

If b = 90, then b = a/sin60 = 2 √ 6/√ 3 = 2 √ 2, and c = bcos60 = √ 2.

3. By Pythagorean Theorem: BD? =AB？ -Advertising? = 13? - 12? =25, so BD=5

DC？ =AC？ -Advertising? = 13? - 12? =25, so DC = 5 (is AC 13? )

BC=BD+DC= 10