X+y=20 is it linear. and is it proportional

What is the point of inflection for the function f(x)=2cos4x on the interval (0, pi/2)

Ateacher wanted to buy a chair, a bookshelf, two tables and a desk. she spent $900 for all five items and the chair and the desk combined 70% of her total. if the bookshelf cost $50, how much did each of the tables cost?

Check all that apply. f is a function. f is a one-to-one function. c is a function. c is a one-to-one function.