A. Golden Ratio:
1. A rectangle has a length that is 1.618 times its width. If the width of the rectangle is 10 cm, what is its length?
Solution: Let the length of the rectangle be x. Then, x/10 = 1.618. Solving for x, we get x = 16.18 cm. Therefore, the length of the rectangle is 16.18 cm.
2. A certain shell spirals outward at a ratio of approximately 1.618 to 1 for each quarter turn. If the shell makes 6 complete turns, how many total spirals are there?
Solution: The number of spirals in each turn is equal to the sum of the number of spirals in the previous two turns. Starting with 1 spiral in the first turn, we get the sequence: 1, 1, 2, 3, 5, 8. Therefore, there are 1+1+2+3+5+8 = 20 spirals in total.
3. The ratio of the length of a line segment to that of its longer part is the same as the ratio of the longer part to the shorter part. If the length of the line segment is 8, what is the length of the longer part?
Solution: Let the length of the longer part be x. Then, 8/x = x/(8-x). Solving for x, we get x = (1+sqrt(5))/2 * 8. Therefore, the length of the longer part is (1+sqrt(5))/2 * 8.
4. The height of a triangle is 1.618 times its base. If the base of the triangle is 10 cm, what is its height?
Solution: Let the height of the triangle be x. Then, x/10 = 1.618. Solving for x, we get x = 16.18 cm. Therefore, the height of the triangle is 16.18 cm.
5. The ratio of the diagonal of a regular pentagon to its side length is the golden ratio. If the side length of the pentagon is 5 cm, what is the length of its diagonal?
Solution: Let the length of the diagonal be x. Then, x/5 = 1.618. Solving for x, we get x = 8.09 cm. Therefore, the length of the diagonal is 8.09 cm.
B. Translating English sentences into mathematical sentences:
1. Twice a number minus four is equal to ten. Translate this into a mathematical sentence.
Solution: Let the number be x. The mathematical sentence is: 2x - 4 = 10.
2. The sum of two numbers is equal to seven times the difference between the numbers. Translate this into a mathematical sentence.
Solution: Let the two numbers be x and y. The mathematical sentence is: x + y = 7(x - y).
3. The product of a number and five is equal to the difference between ten and the number. Translate this into a mathematical sentence.
Solution: Let the number be x. The mathematical sentence is: 5x = 10 - x.
4. The square of a number is equal to the difference between four times the number and six. Translate this into a mathematical sentence.
Solution: Let the number be x. The mathematical sentence is: x^2 = 4x - 6.
5. The quotient of a number and three is equal to the sum of the number and two. Translate this into a mathematical sentence.
Solution: Let the number be x. The mathematical sentence is: x/3 = x + 2.
C. Translating mathematical sentences into English sentences:
1. 3x + 4y = 10. Translate this into an English sentence.
Solution: The sum of three times x and four times y is equal to ten.
2. 2(x - 5) = 8. Translate this into an English sentence.
Solution: Twice the difference between x and five is equal to eight.
3. 5a - 2b > 7. Translate this into an English sentence.
Solution: The difference between five times a and two times b is greater than seven.
4. 2x^2 - 5x + 3 = 0. Translate this into an English sentence.
Solution: The quadratic equation 2x^2 - 5x + 3 is equal to zero.
5. (a + b)^2 = a^2 + 2ab + b^2. Translate this into an English sentence.
Solution: The square of the sum of a and b is equal to the sum of the squares of a and b, plus two times a times b.
D. Compound statements into symbolic form:
1. If it rains, then I will stay indoors.
Solution: Let p be "it rains" and q be "I will stay indoors". The symbolic statement is: p → q.
2. The car will start if and only if the battery is charged.
Solution: Let p be "the car will start" and q be "the battery is charged". The symbolic statement is: p ↔ q.
3. I will go to the beach or the park.
Solution: Let p be "I will go to the beach" and q be "I will go to the park". The symbolic statement is: p ∨ q.
4. If I study hard, then I will pass the exam. If I don't study hard, then I will fail the exam.
Solution: Let p be "I study hard" and q be "I pass the exam". The symbolic statements are: p → q and ¬p → ¬q.
5. The number is even if it is divisible by 2.
Solution: Let p be "the number is even" and q be "the number is divisible by 2". The symbolic statement is: p ↔ q.
E. Symbolic form into compound statements:
1. p ∧ q
Solution: p and q are both true.
2. ¬p → q
Solution: If p is false, then q is true.
3. p ∨ ¬q
Solution: Either p is true or q is false.
4. p ↔ ¬q
Solution: p and q have opposite truth values.
5. (p ∧ q) → ¬r
Solution: If both p and q are true, then r is false.
F. Construction of truth table:
1. p ∧ q
Solution:
p | q | p ∧ q
--|---|------
T | T | T
T | F | F
F | T | F
F | F | F
2. p ∨ q
Solution:
p | q | p ∨ q
--|---|------
T | T | T
T | F | T
F | T | T
F | F | F
3. ¬p
Solution:
p | ¬p
--|---
T | F
F | T
4. p → q
Solution:
p | q | p → q
--|---|------
T | T | T
T | F | F
F | T | T
F | F | T
5. (p ∧ q) ↔ (p ∨ q)
Solution:
p | q | p ∧ q | p ∨ q | (p ∧ q) ↔ (p ∨ q)
--|---|------|-------|-------------------
T | T | T | T | T
T | F | F | T | F
F | T | F | T | F
F | F | F | F | T
G. Inverse, converse, and contrapositive:
1. If it rains, then the ground is wet.
Solution:
- Inverse: If it doesn't rain, then the ground is not wet.
- Converse: If the ground is wet, then it rains.
- Contrapositive: If the ground is not wet, then it doesn't rain.
2. All dogs have four legs.
Solution:
- Inverse: If something doesn't have four legs, then it is not a dog.
- Converse: If something is a dog, then it has four legs.
- Contrapositive: If something doesn't have four legs, then it is not a dog.
3. If a number is divisible by 3, then it is divisible by 9.
Solution:
- Inverse: If a number is not divisible by 3, then it is not divisible by 9.
- Converse: If a number is divisible by 9, then it is divisible by 3.
- Contrapositive: If a number is not divisible by 9, then it is not divisible by 3.
4. All rectangles have four sides.
Solution:
- Inverse: If something doesn't have four sides, then it is not a rectangle.
- Converse: If something is a rectangle, then it has four sides.
- Contrapositive: If something doesn't have four sides, then it is not a rectangle.
5. If a shape is a square, then it is a rectangle.
Solution:
- Inverse: If a shape is not a square, then it is not a rectangle.
- Converse: If a shape is a rectangle, then it is a square.
- Contrapositive: If a shape is not a rectangle, then it is not a square.