Which statement regarding the function y=cos(x) is true?

a. the cosine function is odd, so it is symmetrical across the origin.
b. cos(x)=cos(-x)
c. -cos(x)=cos(x
d. since the cosine function is eevn, reflection across the x-axis does not change the graph.

Hints:
use the calculator to check if any of the options are correct.  Try x=pi/6 (or 30 degrees).

Odd functions satisfy the property
f(x)=-f(-x)
For this reason, odd functions are symmetrical about the origin.
Examples of odd functions: sin(x), x^3, x+3x^5, tan(x), sin(x)*cos(x), x*sin(x)
(use your calculator to check that the above are true).

Even functions satisfy the property
f(x)=f(-x)
Even functions are symmetrical about the y-axis.
Examples: x^2, cos(x), x^4+2, |x|, sin^2(x), x^2*cos(x)

There are functions that are neither odd nor even, 
examples:
x^2+4x+3, sin(x)+cos(x)

First subtract 10 from both side
y^2+6y-16=0
(y+8)(y-2)=0
-8 and 2