a) Ta có phương trình: 7 . 3^x + 20 . 3^x = 3^25
Simplifying the equation, we have: 27 . 3^x = 3^25
So, 3^x = 3^25 / 27
Since the bases are the same, the exponents must be equal. Therefore, x = 25 - 3 = 22.

b) Ta có phương trình: 2/3x - 3/4x = (-5/12) . 8/15
Simplifying the equation, we have: (8/9 - 9/12)x = (-5/12) . 8/15
Combining the fractions on the left side, we have: (32/36 - 27/36)x = (-5/12) . 8/15
Simplifying the equation further, we have: (5/36)x = (-5/12) . 8/15
Multiplying both sides by 36/5, we have: x = (-5/12) . 8/15 . 36/5
Simplifying the equation, we have: x = -8/3

c) Ta có phương trình: (5x - 39) . 7 + 3.5 = 21/2
Simplifying the equation, we have: 35x - 273 + 3.5 = 21/2
Combining like terms, we have: 35x - 269.5 = 21/2
Multiplying both sides by 2, we have: 70x - 539 = 21
Adding 539 to both sides, we have: 70x = 560
Dividing both sides by 70, we have: x = 8

d) Ta có phương trình: (x + 1) + (x + 2) + (x + 3) +....+ (x + 100) = 5750
Simplifying the equation, we have: 100x + (1 + 2 + 3 +....+ 100) = 5750
The sum of the first n natural numbers can be calculated using the formula: n(n+1)/2
So, the sum of the first 100 natural numbers is: 100(100+1)/2 = 5050
Substituting this value into the equation, we have: 100x + 5050 = 5750
Subtracting 5050 from both sides, we have: 100x = 700
Dividing both sides by 100, we have: x = 7

Bài 11: Tính hợp lý
3^3 . 18/5 - 3^3 . 12/5 - 3^3 . 6/5
Simplifying the expression, we have: 27 . 18/5 - 27 . 12/5 - 27 . 6/5
Combining like terms, we have: (486/5) - (324/5) - (162/5)
Subtracting the fractions, we have: (486 - 324 - 162)/5
Simplifying the numerator, we have: 486 - 486 = 0
So, the expression simplifies to: 0/5 = 0