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If 0 lt alpha lt (pi)/(2) and sin alpha ...

If `0 lt alpha lt (pi)/(2) and sin alpha + cos alpha = sqrt2,` then the value of `cos 3 alpha` is-

A

`(1)/(sqrt2)`

B

`-(1)/(sqrt2)`

C

`1/2`

D

`-1/2`

Text Solution

Verified by Experts

The correct Answer is:
B
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Knowledge Check

  • If tantheta =(sin alpha - cos alpha)/(sin alpha = cos alpha) , and (sin alpha+ cos alpha ) = k costheta , then the value of k is

    A
    2
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  • If A=[(cos alpha, -sin alpha),(sin alpha, cos alpha)], and A+A'=I , then the value of alpha is

    A
    `(pi)/(6)`
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    D
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