# Math Help - trig identity

1. ## trig identity

hey all this one question keeps bugging me if you could help thanks so much!
if it is an identity prove it.
(sinX+cosx)^2=1

if i expand it it becomes
sinx^2+cosx^2+2sinxcosx=1

if i chose to changd sinx^2 to 1-cos^2 it becomes

(1-cosx^2)+cosx^2+sinxcosx=1
so it simplifies to
1+sinxcosx=1
I cant seem to go anywhere from there....if im on the right track please let me know! or is it just not an identity?
THANKS!

2. Originally Posted by amanda0603
hey all this one question keeps bugging me if you could help thanks so much!
if it is an identity prove it.
(sinX+cosx)^2=1

if i expand it it becomes
sinx^2+cosx^2+2sinxcosx=1

if i chose to changd sinx^2 to 1-cos^2 it becomes

(1-cosx^2)+cosx^2+sinxcosx=1
so it simplifies to
1+sinxcosx=1
I cant seem to go anywhere from there....if im on the right track please let me know! or is it just not an identity?
THANKS!
so in other words, you have 1 = 1 + something

and the something is not always 0. does that seem like an identity?