Angle 1 and Angle 5 are complementary because the sum of both of those angles equals 90 degrees and the other 3 angles cannot be complementary to each other because they are already 90 degree angles by themself
You can see that all of the other angles are exactly 90 degrees. The remaining two are angles 1 and 5 which means that they have to be 90 degrees. Since they are together, they also look perfectly like a 90 degree angle.
