My 7th grader has enough mathematical background in algebra, geometry and trigonometry to start learning how to write out proofs. Are their good books that teach this step by step? I can certainly teach him myself with examples, but I figured there must be a systematic way to teach this. I just searched on Amazon and found a few books with good reviews, but any recommendations would be great! Given the age of the student, the book should start off at a simple level and then go from there. Thanks!