>The book I'm reading gives the algorithms for adding and subtracting >fractions: > >a/b+c/d=ad+bc/bd >and similarly for subtraction. It notes that this "does not necessarily >give you the LCD". They do mention 4 methods for obtaining the LCD "if >desirable". That "if desirable" phrase appears three times. It never >mentions *how* desirable finding the LCD is, and the whole section on it >seems to be an afterthought. >Given the emphasis on calculators in the book, I'm wondering if the LCD >is still an issue, or is it considered nice to know, but don't spend >alot of time having child memorize how to find this. What is the current >thinking on LCD and does it come up on the standardized testing everyone >is barking about? >I have no memory of being taught fractions, and, when I put myself to a >set of them, I found I used a combo of good guess and reduction after >calculating, with a pinch of prime numbers sprinkled over the good >guess. Not a method given to methodical instruction. >TIA >blacksalt > >--
Lowest Common Denominator is an important concept for knowledge and skill development. Using LCD often helps in keeping arithmetic steps simpler, making simplifications later in the final answer easier.
The algorithm you quote is used exactly as you quoted only when the denominators, b, and d, have no common factor; when they do have a common factor, you may find a Lowest Common Denominator, but you can yet use the algorithm which you quoted in order to obtain a Common Denominator, which will still work.
Relax! Arithmetic is still arithmetic. With Algebra, at least you understand the rules more clearly.