As shown in the figure, if we know ad ‖ BC, we need to add a condition to prove △ ABC ≌ △ CDA according to SAS______ ．

In △ ABC and △ CDA, AC = Ca, DAC = bcaad = CB, and △ ABC ≌ CDA (SAS), so the answer is: ad = CB

What does arctan30 mean when it often appears in math problems

When you find the angle a, you know that when Tana = 30, you can't find the specific value of the angle, so you use arctan30 to represent the angle A

Give me some interesting math questions

The lame fox saw that the old goat was selling green onions. He went over and asked, "old goat, how do you sell green onions? How many green onions are there?"
The old goat said, "one kilogram of scallion costs one yuan, a total of 100 kilograms."
The lame fox turned his eyes and asked, "how white are your scallions, and how many are their leaves?"
Old goat quite impatiently said: "a scallion, scallion white accounted for 20%, the other 80% are scallion leaves."
The lame fox broke his fingers and said, "onion white, I'll give you seventy cents for one kilogram. Onion leaf, I'll give you thirty cents for one kilogram. Seventy cents plus thirty cents is just one yuan, OK?"
The old goat thought about it and thought that the fox had a point, so he agreed to sell it to him. The fox laughed and began to count the money
The fox first made a formula:
0.7 × 20 + 0.3 × 80 = 14 + 24 = 38 (yuan), then said: "100 kg of scallion, scallion white accounts for 20%, that is 20 kg. Scallion white 1 kg is 7 Jiao, that is 14 yuan in total; scallion leaf accounts for 80%, that is 80 kg, 1 kg is 3 jiao, that is 24 yuan in total. Together, that is 38 yuan, right?"
After a long time of calculation, the old goat couldn't figure out a number, so he had to say, "you're right."
"I've never been cheated by a fox! Here's 38 yuan. Count it!" the fox handed the money to the old goat. When the old goat went home after selling green onions, he always felt that the money was a little less, but where was the less? He looked down and saw the little mole come out of the field. He asked the little mole to help calculate the account
The little mole said, "you used to sell green onions for 1 yuan per kilogram. You have 100 kilograms, so you should sell them for 100 yuan. Why does the lame fox only give you 38 yuan?"
The old goat nodded and knew that he had suffered a loss. But he didn't understand how he had suffered a loss?
The mole said, "the fox gives you 7 cents for a kilogram of scallion and 3 cents for a kilogram of scallion leaves. It's only 1 yuan for two kilogram. You've already suffered half of the loss."
The old goat asked, "if I suffer half losses, I should get 50 yuan. Why only 38 yuan?"
Mole wrote a formula:
(1-0.7) × 20 + (1-0.3) × 80 = 6 + 56 = 62 (yuan), "you lose 0.3 yuan for 1 kg scallion, 6 yuan for 20 kg scallion, 0.7 yuan for 1 kg scallion leaf, 56 yuan for 80 kg scallion leaf. All in all, you lose 62 yuan."
The old goat turned around and ran back. He saw that the fox was selling green onions for 2 yuan per kilogram. Without saying a word, the old goat propped the lame Fox's back with its horns and pushed him into the pond

Find ten interesting math questions for grade four to five
As above, ten interesting questions. Less words are preferred
I want more math, not IQ. But mathematics is very interesting.

Xiaohua's father can cut five nails in one minute. How many nails can he cut in five minutes?
2. Xiaohua took 50 yuan to the store to buy a 38 yuan car, but the salesman only gave him 2 yuan. Why?
3. Xiaojun said: "I went fishing yesterday and caught a tailless fish, two headless fish and three half fish. Guess how many fish I caught in total?" students guess how many fish Xiaojun caught in total?
How many miles did each horse run? How many miles did the six horses run in total?
5. A dog tied to a tree trunk is greedy for a bone on the ground, but the rope is not long enough, which is 5 cm short. Can you teach the dog how to grasp the bone?
6. Wang went from place a to place B. one minute later, Li went from place B to place A. when Wang and Li met on the way, which one was farther from place a?
7. The clock has just struck 13. What should you do now?
8. In the vast grassland, there is a cow eating grass. This cow only eats half of the grass in a year. How many years will it take to eat all the grass in the grassland?
9. Mother has seven pieces of candy, she wants to give them to three children equally, but she doesn't want to cut the rest of the candy. What can mother do?
10. There is a row of trees on the road of the park. Each tree is 3 meters apart. How many meters are there between the first tree and the sixth tree?
answer:
20, including fingernails and toenails
2. Because he paid the salesman 40 yuan, he only gave him 2 yuan;
3.0, because the fish he catches doesn't exist;
4.6 Li, 36 Li;
5. Just teach the dog to turn around and scratch the bone with his back foot
6. When they met, they were in the same place, so they were as far away from Jiadi;
7. The clock should be repaired;
8. It will never eat up the grass, because it will keep growing;
9. Mother should eat one piece first, and then give each child two pieces;
15 m;

As shown in the figure, if we know ad ‖ BC, we need to add a condition to prove △ ABC ≌ △ CDA according to SAS______ ．