Which equation is a quadratic equation?

3x − y = 8x + 2

y = 7x(5 − 2x) + 9

7x2 + y = (3 + x)(7x − 1)

y − 3 = (x2 − 3)(8x + 3)

Question

Which equation is a quadratic equation? 

 3x − y = 8x + 2 

 y = 7x(5 − 2x) + 9 

 7x2 + y = (3 + x)(7x − 1) 

 y − 3 = (x2 − 3)(8x + 3)

whpalmer4 · Answer

A quadratic equation in $x$ is an equation in terms of $x^2, x^1, x^0$.  If you expand those equations above, which of them can be written in the form $y=ax^2+bx+c$ where $a,b,c$ are constants?

anonymous · Answer

y = 7x(5 − 2x) + 9 this one

whpalmer4 · Answer

That's correct!

whpalmer4 · Answer

The first one doesn't have an $x^2$ term, the third one the $x^2$ terms cancel out, and the final one has an $x^3$ term