`cos3theta+costheta=0`
`rArr cos3theta=-costheta=cos(pi-theta)`
`rArr 3theta=2npi+(pi-theta)` or `3theta=2npi-(pi-theta)`
`rArr 4theta=2npi+pi` or `2theta=2npi-pi`
`rArr 4theta=(2n+1)pi` or `2theta=(2n-1)pi`
`rArr theta =(2n+1)pi/4` or `theta=(2n-1)pi/2`. Ans.
