## MTALHAHASSAN2 one year ago Need help!! 2. Give that cos (x+y) = cos x cos y – sin x sin y, use this to express cos 2x in terms of cos x

1. campbell_st

so replace y with x so you have cos(x + x) = cos(x)cos(x) - sin(x)sin(x) and simplify from there.

2. campbell_st

so the left side of the equation becomes cos(2x)

3. MTALHAHASSAN2

why do you do that

4. campbell_st

now you can simplify the right side

5. MTALHAHASSAN2

I mean why do you change the y into x

6. campbell_st

that way when you replace y with x cos(x + y) = cos(x + x) which is cos(2x)

7. MTALHAHASSAN2

|dw:1437707497195:dw|

8. MTALHAHASSAN2

right

9. campbell_st

thats correct.... but the question asks in terms of cos(x) so what do you know about sin^2(x)..?

10. MTALHAHASSAN2

sin2x = 2sinx cosx

11. campbell_st

no its $\sin^2(x)$

12. MTALHAHASSAN2

is there is any formula for this

13. MTALHAHASSAN2

I don't have it for this one I have for Sin2x

14. campbell_st

sure its one of the fundamentals $\cos^2(x) + \sin^2(x) = 1$

15. MTALHAHASSAN2

oh know it be sin^2 (x) = 1-cos^2x

16. MTALHAHASSAN2

= cos^2 x -(1-cos^2 x)

17. MTALHAHASSAN2

@campbell_st

18. MTALHAHASSAN2

@mathway