Answers to 9 questions on page 27 of mathematics book of grade two

I made it myself. I don't know if it's right~
∵∠BCE+∠ECA=90°
∠BCE+∠BCE=90°
∴∠ECA=∠BCE
∵∠DCA+∠EBC=90°
∠DCA+∠DAC=90°
The three angles of △ EBC and △ CDA are corresponding here~

RELATED INFORMATIONS

1. 1. Which of 9, 2, 11, 97, 51, 12, 91, 27, 39, 4 are prime numbers? Which are composite numbers?
2. Fill in the numbers according to the rules: 51.2,64,80, (), ()
3. 51.2,64,80, (), () find the rule and fill in the number. How to write? And (), (), 19.8,13.2,8.8 also find the rule and fill in the number, how to write?
4. Find the rule 51.2,64,80100, () don't call me Baidu, because Baidu doesn't have it
5. 5. 7, 11, 15, 23 infer according to the sequence law. What's next? A、26 B、27 C、28 D、29
6. Sequence reasoning 1.-27,-64,-125,（） 2.2,12,36,80,（） 3.16,8,8,12,24,60,（） 4.4,3,2,0,1,-3,（） 5.4,11,30,67,（） 6.8,10,13,（）,25,26 7.24,6,4,（）,2,0
7. Sequence derivation 2 3 10 15 26 ( )
8. The derivation method of this sequence is The sum formula of the first n terms of the arithmetic sequence {an} is_____ The derivation method is as follows_______ The general formula of equal ratio sequence {an} is_____ The derivation method is as follows_______ The sum formula of the first n terms of the equal ratio sequence {an} is_______ The derivation method is as follows________ I just want to know what the derivation method is, so you just fill in the derivation method
9. Derivation of sequence problem Sn = 1 / A1A2 + 1 / a2a3 +. + 1 / Anan + 1 for Sn
10. Find the rules and fill in the numbers ① 1, 1, 2, 3, 5, 8______ ②0.625，1.25，2.5，5，______ ．
11. 6 questions on page 23 of the mathematics textbook for the first semester of the second grade of the people's Education Press 11.3, 6 questions
12. How to simplify the operation of 1 + 10 + 4 (10 / 35) + 2 (24 / 63) + 1 (51 / 99) + 1 (7 / 143)
13. Simple calculation of 0.865 * 51% + 51 * 35.5% / 2 * 1 / 50 + 4.9% * 12 and 1 / 5 There must be a process! Online and so on!
14. 1. 5 + 3 = 8; 8 = 56 / 7; 56-5 = 51. Is the answer correct? 2. Xiao Hua does an integer addition problem, regarding 3 as 8 and 9 as 6. The sum is 356. What's the correct answer? 356-68 = 288; 288 + 93 = 381
15. 56×165÷(7×11)+144×13÷156 (28.8＋28.5＋28.7＋28.1＋28.6）÷5 41.2×8.1＋11×9.25＋537×0.19 Who is the quickest to give the reward! 1. Of course, I know the answer,
16. The greatest common factor of 20 and 15, 36 and 9, 51 and 34, 18 and 90
17. 2 5 6 11 23 () () find the law
18. Find the rule 2,5,11,23, (), ()
19. Find the rules and fill in the numbers: 1250, 20625, 17125, 14, (), (), (), ()
20. Using the sum of general term, 1 + 11 + 111 + +111… The sum of 1n ones