It is known that in RT triangle ABC, D is the midpoint of hypotenuse AB, de / / BC, EF / / DC

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• 1. In the triangle ABC, extend AC to point F so that CF = 2 / 1Ac, D and E are the midpoint of edge AB and BC respectively. Prove DC = EF
• 2. It is known that x = 2, y = 1 is the solution of the binary linear equations ax + by = 7, ax by = 1 about x.y=————
• 3. Calculation: 5x-2 [3x-2 (3x + 1)]
• 4. Known: x ^ 2-3x + 1 = 0, find: algebraic formula x ^ 2 + 1 / x ^ 2 and x ^ 4 + 1 / x ^ 4 value
• 5. Given x ^ 2-3x + 1 = 0, find the value of algebraic formula x ^ 4 + 1 / x ^ 4 Such as the title
• 6. Given x ^ 2-x-1 = 0, find the value of the algebraic formula x ^ 3-x ^ 2-3x + 2008
• 7. When x is equal to, x + 1 is equal to x minus 1 RT
• 8. X minus one part of X is equal to t, and the algebraic expression containing T is used to express X I mean, for example, if x minus one equals T, then x equals t plus one
• 9. Given a + B = 5, ab = 3, then the value of the algebraic formula a3b-2a2b2 + AB3 is______ ．
• 10. Given a + B = 23, ab = 2, find the value of the algebraic formula A2B + 2a2b2 + AB2
• 11. As shown in the figure, in RT △ D is the midpoint of the hypotenuse AB, f is the midpoint of AC, EF ‖ DC, intersects the extension line of BC at point E, and proves that the quadrilateral befd is isosceles trapezoid
• 12. It is known that the area of triangle ABC is 42.3 square centimeters, which is six times of the area of triangle EFB, and the area of parallelogram efcd
• 13. In triangle ABC, CD is the angular bisector, CF is the outer angular bisector, DF is parallel, BC intersects AC and E, CF intersects F, and de = EF is proved
• 14. In the acute triangle ABC, ah is high, if AB + BH = HC, prove: angle B = 2, angle c (to process,) In the acute triangle ABC, ah is high, if AB + BH = HC, prove: angle B = 2, angle C
• 15. As shown in Figure 8, fold triangle ABC along De, when point a falls inside the quadrilateral BCDE As shown in the figure, when the triangle paper ABC is folded along De, when point a falls inside the quadrilateral BCDE, there is a quantitative relationship between a and ∠ 1 ＋ 2, which always remains unchanged. Please find out this rule? And write down the reasons
• 16. As shown in the figure, fold the triangle ABC paper along De, when point a falls inside the quadrilateral BCDE, ∠ a, ∠ 1, ∠ 2 When point a falls in the interior of the quadrilateral BCDE, what is the quantitative relationship among the degrees of ﹥ a, ﹥ 1, ﹥ 2? Please write it down and explain why
• 17. As shown in the figure, fold the paper triangle ABC along de. when point a falls inside the quadrilateral BCDE, what is the relationship between angle A and angle 1 plus angle 2? Explain the reason
• 18. As shown in the figure, fold the triangle paper ABC along de. when point a falls on the inner A1 of the quadrilateral BCDE, if ∠ 1 = 50 °∠ 2 = 20 °, calculate the degree of ∠ A1
• 19. As shown in the figure, fold the paper △ ABC along De, point a falls at a ′, ∠ 1 + ∠ 2 = 150 °, then ∠ a=______ ．
• 20. As shown in the figure, when the triangle ABC paper is folded along De, and point a falls inside the quadrilateral BCDE, the quantitative relationship between ∠ A and ∠ 1 + 2 is