1.50

2.AB=AC or ∠B=∠C or BD=CD.

isosceles

1. One base angle of the isosceles triangle is equal to or greater than 90 degrees.

5.C

6.B

7. Tip: ∠B=∠C=∠DEB

8. Measure whether BD and CD are equal ∠ ADB = 90 or ∠B and ∠C to see if they are equal.

9. Known: △ABC, verification: There cannot be two proofs of right angles in ∠A, ∠B and ∠C: Let ∠A, ∠B and ∠ C have two right angles, and let ∠ A = ∠ B = 90. This is in contradiction with the theorem of triangle interior angle sum. Therefore, two right angles in ∠A, ∠B and ∠C are not true, so two angles in a triangle cannot be right angles.

10. Known: 1

3 (or 1

Four or two?

3, or 2

4)

Proof: ellipsis

1 1( 1)△ABC, △BDF, △EFC, △BFC, △ Ade

(2) Because AB=AC, ∠ABC=∠ACB.

Because DE is parallel to BC, BF bisects ∠ABC, ∠DFB=∠CBF=∠ABF.

So DB=DF.

Similarly, EF=EC.

So the C triangle ade = AD+AE+DE = AD+AE+DF+EF = AD+AE+AD+EC = AB+AC.

(3) Establish