It is known that: as shown in the figure, in △ ABC, ∠ BAC = 120 ° AB = AC, the vertical bisector on the side of AB intersects BC at point D, intersects AB at point E, and connects ad, Verification: CD = 2bd

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• 1. As shown in the figure, in the isosceles △ ABC, ab = AC, ∠ BAC = 120 °, ad is the height on the edge of BC, passing through point d to make de ‖ AB, intersecting AC at point E. in addition to △ ABC, is there any isosceles triangle in the figure? If yes, please point out and give reasons
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• 3. Calculation: 19492-19502 + 19512-19522 + +19972-19982+19992=______ ．
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• 11. As shown in the figure, D is any point on the bottom edge BC of the isosceles triangle ABC, take a point E, AE = ad on the ray AC, and verify the angle bad = 2 EDC
• 12. If P is a point outside the plane where △ ABC is located and the lines AP, BP and CP are perpendicular, then the projection of P on plane ABC is ()
• 13. It is known that △ ABC is inscribed in ⊙ o, and ab is the diameter of ⊙ O. the bisector of ∠ ACB intersects ⊙ o at point D. if AB = 2cm, then ad=______ cm．
• 14. It is known that: as shown in the figure, △ ABC is the inscribed triangle of ⊙ o, ⊙ O's diameter BD intersects AC at e, AF ⊥ BD at F, extending AF intersects BC at g. the proof is AB2 = BG · BC
• 15. It is known that: as shown in the figure, △ ABC is the inscribed triangle of O, the bisector of angle ACB intersects circle O at point D, and makes tangent l of circle O through point D. prove that AB is parallel to L
• 16. AB is the diameter of the circle O, Pb tangents the circle O to B, D on the circle O, ad ‖ Po, prove that PD is the tangent of the circle o
• 17. As shown in the figure, AB is the diameter of ⊙ o, Pb is tangent to ⊙ o at point B, the extension line of chord AC ∥ OP and PC intersects BA at point D, proving that PD is tangent to ⊙ o
• 18. EA is the tangent line of circle O, a is the tangent point, the chord BC intersects OA at D, through B makes Pb, vertical CB intersects EA extension line at P
• 19. In the eighth grade mathematics equilateral triangle ABC, there is a point P, AP is equal to 3, BP is equal to 1, CP is equal to 5?
• 20. As shown in the figure, given that points B, C and D are on the same straight line, △ ABC and △ CDE are equilateral triangles. Be intersects AC at F, ad intersects CE at h. ① verify: △ BCE ≌ △ ACD; ② verify: CF = ch; ③ judge the shape of △ CFH and explain the reason