Posted by **Solly** on Tuesday, May 10, 2011 at 2:48am.

Cos70cos10+sin70sin10 e =2cos^2 2x-1

- Maths -
**Reiny**, Tuesday, May 10, 2011 at 7:24am
Cos70cos10+sin70sin10 e =2cos^2 2x-1

don't know what that "e" is supposed to be, seems to be superfluous.

recall : cos (A-B) = cosAcoB + sinAsinB

Cos70cos10+sin70sin10 =2cos^2 2x-1

cos(70-10) = 2cos^2 2x - 1

cos60 = 2cos^2 2x - 1

recall cos 2A = 2cos^2 A - 1

so 2cos^2 2x - 1 = cos 4x

cos 4x = cos60

4x = 60° or 4x = 300°

x = 15° or x = 75°

since cos 2x has a period of 180°

other solutions would be

x = 15+180 = 195°

or

x = 75+180 = 255°

## Answer This Question

## Related Questions

- Math - Determine the solution of the equetion: 2cos x-1=0? 2cos x=0? and 2cos x+...
- math - cosA= 5/9 find cos1/2A are you familiar with the half-angle formulas? the...
- Maths - y = -14sin(8x)2cos(5x) - 4sin(5x)cos(8x) u = -14sin(8x) u' = -112cos(8x...
- Maths - How do you derive: 2cos(3-2x)?
- maths - solve for x if 2cos(x-30)=1, £(-90;270)
- Maths - solve the following eq. [0<=x<=360] 2cos(3x-72)=1
- maths - 2cos^2a*cos^2b+2sin^2asin^2b-1=cos2acos2b
- Maths - y = 2[cos1/2(f1 - f2)x][cos1/2(f1 + f2)x] let f1 = 16 let f2 = 12 ...
- Trigonometry - Need help solving problems using the Trigonometric Identities: ...
- maths - give the solution of the initial-value problem dy/dx=(1+2cos^2(x))/y, (y...

More Related Questions