Prove, using the second derivative, that the general quadratic y= ax^2+bx+c, is:
a) always convex when a>0
b) always concave when a <0

International calls require the use of a country code. many country codes a 3-digit numbers. country codes do not begin with a 0 or 1. there are no restrictions on the second and third digits. how many different 3-digit country codes are possible?

Which of the following is an equation of a line parallel to the equation y=4x+1?

Through which points should a line of rotation be placed to create a cylinder with a radius of 3 units