Saturday

February 28, 2015

February 28, 2015

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

- Maths -
**Reiny**, Tuesday, May 10, 2011 at 7:24amCos70cos10+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 - cosA= 5/9 find cos1/2A are you familiar with the half-angle formulas? the...

Maths - solve the following eq. [0<=x<=360] 2cos(3x-72)=1

maths - 2cos^2a*cos^2b+2sin^2asin^2b-1=cos2acos2b

maths - give the solution of the initial-value problem dy/dx=(1+2cos^2(x))/y, (y...

maths - give the solution of the initial-value problem dx/dy=(1+2cos^2(x))/y, (y...

Trigonometry - Need help solving problems using the Trigonometric Identities: ...

trig - Find minimum value of 2cos x + 2cos y

trig - Find minimum value of 2cos x + 2cos y

trig - Find minimum value of 2cos x + 2cos y

math - the problem is 2cos^2x + sinx-1=0 the directions are to "use an identity ...