1.

Let `f(n)=2cosn xAAn in N ,`then `f(1)f(n+1)-f(n)`is equal to`f(n+3)`(b) `f(n+2)``f(n+1)f(2)`(d) `f(n+2)f(2)`A. `f(n+3)`B. `f(n+2)`C. `f(n+1)f(2)`D. `f(n+2)f(2)`

Answer» Correct Answer - B
`f(n)=2cos nx`
`rArr f(1)f(n+1)-f(n)`
`=4cos x cos(n+1)x-2cos nx`
`=2[2cos (n+1)x cos x-cos nx]`
`=2[cos(n+2)x+cos nx-cosnx]`
`=2cos(n+2)x=f(n+2)`.


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