Analyzing the Equation and the Range
Understanding the Equation:
We can factor the equation as follows:
n = ab + 2a + 3b
n = a(b + 2) + 3b
n = a(b + 2) + 3(b + 2) - 6
n = (a + 3)(b + 2) - 6
Implications of the Equation:
This means that n + 6 must be a product of two integers greater than 2.
Considering the Range:
We're looking for n between 70 and 90, inclusive.
So, n + 6 will be between 76 and 96, inclusive.
Finding Suitable Pairs
We need to find pairs of integers greater than 2 whose product lies between 76 and 96.
Lower Bound: The smallest product we can form with integers greater than 2 is 3 * 4 = 12. This is far below 76.
Upper Bound: We can start checking larger products.
5 * 16 = 80
6 * 13 = 78
7 * 12 = 84
8 * 11 = 88
9 * 10 = 90
Counting Valid Values of n:
For each of the products above, we can find a corresponding n by subtracting 6.
Therefore, there are 5 possible values of n between 70 and 90 that can be expressed in the given form.
Answer: There are 5 integers n with 70 ≤ n ≤ 90 that can be written as n = ab + 2a + 3b for at least one ordered pair of positive integers (a, b).