### If cos θ + cos2θ = 1, then find the value of expression (sin2θ + sin4θ).

1

Step by Step Explanation:
1. It is given that
cos θ + cos2θ = 1 ............. Eq. (1)
2. Above equation can be re-written as following
⇒ 1 - cos2θ = cos θ
3. Since 1 - cos2θ = sin2θ, we can re-write equation as following,
⇒ sin2θ = cos θ ............. Eq. (2)
4. Now replace value of sin2θ = cos θ, in required expression
sin2θ + sin4θ = cos θ + cos2θ
⇒ sin2θ + sin4θ = 1 .................... Using Eq. (1)