# Finding zeroes for trig functions

• April 17th 2008, 05:26 PM
theowne
Finding zeroes for trig functions
When you have functions like these:

y=sinx+cos2x
or
y=-sinx-4cos2x

How can I determine the zeroes? Is there an identity for cos2x that could be useful in factoring a term out, or etc?
• April 17th 2008, 05:35 PM
Jhevon
Quote:

Originally Posted by theowne
When you have functions like these:

y=sinx+cos2x
or
y=-sinx-4cos2x

How can I determine the zeroes? Is there an identity for cos2x that could be useful in factoring a term out, or etc?

write $\cos 2x$ as $1 - 2 \sin^2 x$. then your equations break down into quadratic equations in $\sin x$. It is easy to find the zeros from there