Question

cos(n + 1) a.cos(n - 1) a + sin(n + 1) a.
sin(n − 1) a =
1. cos 2n a
2. sin 2na
3. cos 2a
4. sin 2a​

Answer 1

Answer:

• Let, A be (n+1)a and B be (n-1)a
• Therefore,
• CosA .CosB + SinA .Sinb
• Cos [A+B] = Cos [ (n+1)a +(n-1)a]
• = Cos [ na +a - na +a]
• therefore ans is cos2a