If 1 + cosxcosx - 2sin^2xsin2x = 0 , what is the value of xx?

1 Answer
Jun 11, 2016

x = { pm pi +2k pi} uu {pm pi/3 + 2k pi} x={±π+2kπ}{±π3+2kπ} for k = 0,pm1,pm2,pm3,...

Explanation:

Making sin(x)^2=1-cos(x)^2 and y = cos(x) we obtain

1+y-2(1-y^2)=0

Solving for y we have {y=-1,y=1/2} so the solutions for x must obey

cos(x) = -1 or cos(x) = 1/2

Solving for x we obtain

x = pm pi +2k pi for cos(x) = -1 and
x = pm pi/3 + 2k pi for cos(x) = 1/2
for k = 0,pm1,pm2,pm3,...