Pre-Algebra: Chapter 9-4
Using Proportions
You can use a proportion to solve problems that relate to ratio. You can use the criss-cross method to solve these type of problems, a/b = c/d or criss-crossing, we can say that a*d = b *c. The book calls these things “cross products” and I just cross my hands to give students a hint.
We can use cross products to solve things like c/35 = 3/7 or criss-crossing, we have c * 7 = 35 * 3
or 7c = 105, dividing by 7 on each side, we have c = 105/7 or c = 15
Here is another explanation from Purplemath too!
Algebra: Chapter 10-4
Addition and Subtraction with Like Denominators
With like denominators, all we have to do is add, simplify and factor the numerators. Just make sure that the denominators are THE SAME. Combine necessary terms, adding only those things that have the same VARIABLES BUT ONLY WITH THE SAME EXPONENTS, combine things that you can and use the techniques to find perfect squares or use the BOX METHOD to factor and see if terms will cancel out. This stuff is easy compared to tomorrow’s stuff. Make sure you know what to do!