Saturday

April 18, 2015

April 18, 2015

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

= 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 11:26pmTry 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**

Math, please help - Which of the following are trigonometric identities? (Can be...

Math integrals - What is the indefinite integral of ∫ [sin (π/x)]/ x^...

Math - Evaluate *Note - We have to find the exact value of these. That I know to...

Trig Help! - Question: Trying to find cos π/12, if cos π/6 = square ...

Precalculus - Use one of the identities cos(t + 2πk) = cos t or sin(t + 2&#...

Pre-Calc : Verify the Identity - Verify the Identity: sin(x+π)/cos(x+3π...

product of function - Given: f(x)=2 cos x and g(x)=sin x. Which of these ...

Math 2nd question - Express as a single sine or cosine function (note: this is ...

TRIGONOMETRY - Can you check my work, please? This is for a trig study guide for...

trig - I need to prove equality. a) (sina + cosa)^2 -1 / ctga - sinacosa = 2tg^...