I'm working through Spivak's Calculus textbook on Chapter 1, 19d.
The question posed is as follows:
Now the part I'm lost on is 19d. I was able to find the only when cases for parts a and b, but now I'm lost for part c. How do I find the case that equality holds only when $x_1 = (lambda)(y_1)$ and $x_2 = (lambda)(y_2)$?
A hint is fine, I'm not just looking for an answer. I'm just really stuck on this one. I have a feeling it has something to do with the terms $(x_1y_1 + x_2y_2)^2$ or $(x_2y_1 - x_1y_2)^2$ but I'm not sure.
Thank you ahead of time.