What is the proof of theorem 2.23, where (b/a)+(d/c)=[(bc)+(ad)]/(ac). Thank you :)
You change nothing when you multiply by 1. So,
b/a + d/c = b/a * c/c + d/c * a/a = bc/ac + ad/ac = (bc+ad)/ac
b/a + d/c = b/a * c/c + d/c * a/a = bc/ac + ad/ac = (bc+ad)/ac