"varies directly" means that you have two numbers or terms that are directly related such as ax = by where a varies directly with b, "varies indirectly" means x = 1/y where x varies indirectly with y.
"Cube of" = multiply number by itself 3 times (3*3*3 = 3^3 three to the cube power)
"cube root of " - similar to square root of but instead of square root, you take the cube root, example CUBE ROOT (27) = 3 (since 3*3*3 = 27).
If, when: f (x) = 5/x IF x DOES NOT EQUAL 0, f(x) has a domain of all real numbers except x=0.
Example: if x>0, then y=2, so this would be part of a segment function where y=2 (horizontal line at y=2) for all values of x greater than zero. Another part of the function could be If x < 0, then y=1
If, Then combinations are also used extensively in computer programs,
What is, when.... example f(x) = x^2 + 5, WHAT IS F(X) WHEN X=3, so substitute 3 for x: (3)^2 + 5 = 9 + 5 = 11.
Let me know if you have any other questions about algebra terms, definitions, etc.