A linear equation is any equation that can be written in the form
ax + b = 0
where a and b are real numbers and x is a variable. This form is sometimes called the standard form of a linear equation. Note that most linear equations will not start off in this form. Also, the variable may or may not be an x so don’t get too locked into always seeing an x there.
The standard form of a quadratic equation looks like this:
ax^2 + bx + c = 0
where a,b , c are known values and x is the variable or unknown (we don't know it yet). Also,a can't be 0.
π(Pi) times the radius (r) squared:
A = π × π r^2
or, when you know the diameter (D):
A = (π / 4) × D^2
or, when you know the circumference (C):
A = C^2/ 4 × π
The intersection of two sets A and B, denoted by A ∩ B, is the set of all objects that are members of both the sets A and B. In symbols,
A ∩ B = { x : x ∈ A and x ∈ B }
That is,x is an element of the intersection A ∩ B if and only if x is both an element of A and an element of B.