Homework Help: Math help again

Posted by Kiely on Thursday, November 29, 2007 at 11:05pm.

cos(3π/4+x) + sin (3π/4 -x) = 0
= cos(3π/4)cosx + sin(3π/4)sinx + sin(3π/4)cosx - cos(3π/4)sinx
= -1/sqrt2cosx + 1/sqrt2sinx + 1/sqrt2cosx - (-1/sqrt2sinx)
I canceled out -1/sqrt2cosx and 1/sqrt2cosx
Now I have
1/sqrt sinx + 1/sqrt2 sinx

And that doesn't equal 0. So where did I go wrong?

Also cos(x+y)cosy + sin(x+y)siny = cosx
I ended up with
(cosxcosy) + sinxsinycosy + (sinxcosy) + cosxsin^2y

I don't know what to do next.

Math help again - Michael, Thursday, November 29, 2007 at 11:26pm
Try your sum and difference identities again. I don't believe you've expanded them correctly.

cos(x + y) = cosx * cosy - sinx * siny
cos(x - y) = cosx * cosy + sinx * siny

sin(x + y) = sinx * cosy + cosx * siny
sin(x - y) = sinx * cosy - cosx * siny

Answer this Question

Related Questions

Trig Help! - Question: Trying to find cos π/12, if cos π/6 = ...
Precalculus - Use one of the identities cos(t + 2πk) = cos t or sin(t...
Math - Evaluate *Note - We have to find the exact value of these. That I know to...
Pre-Calc : Verify the Identity - Verify the Identity: sin(x+π)/cos(x+3&...
Math integrals - What is the indefinite integral of ∫ [sin (&#960...
product of function - Given: f(x)=2 cos x and g(x)=sin x. Which of these ...
TRIGONOMETRY - Can you check my work, please? This is for a trig study guide for...
Math 2nd question - Express as a single sine or cosine function (note: this is ...
Trig - The set of solution(s) of 2 cos^2 (θ) - 13 cos(θ) + 6...
trig - I need to prove equality. a) (sina + cosa)^2 -1 / ctga - sinacosa = 2tg^...

For Further Reading

First Name:
School Subject:
Answer: