|Click on the picture to expand!|
The first thing I did was re-write all of my parameters on paper. I know that I'm looking for the positive integers a, b, c, and d that fit all of these equations. Next, I wrote out equvalent fractions for the (a/c) = (b/d) = (3/4) requirement. I realized that each of these are 2/3rds of a Pythagorean Triple, which fit well with the square root portion of the equations.
Next was time to determine which integers from the fractions to use for each variable, so I listed out the first few Pythagorean Triples of the form x2 + y2 = z2.
I knew from the given equations that I'm looking for a pair Pythagorean Triples whose z- value difference is 15. I chose the triples (of form x2 + y2 = z2) where z = 5, and where z = 20. I then decided which integer from the Pythagorean Triples would represent each of my variables from the problem. The pair a,c had to be from the triple with larger values, and c had to be the larger of those two.
Then, after quickly confirming my variables in the 2nd to last line of the problem, I plugged my variables into the equation ac + bd - ad - bc to find my final answer: 108.
Have a great Monday everyone, and as always, please leave a comment if you have any questions about my solution.
UPDATE: One of our followers on Twitter sent us a picture of his geometric solution to the problem. It's always interesting to see the same problem done in a different way. Check it out below, and thanks, Alex!