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From the double angle formula for cosine, we know that cos(2x)=cos²(x)-sin²(x). Also, we know that sin²(x)+cos²(x)=1. So by substituting the second formula ...

(3* square root of 2) square=  (3* square root of 2) * (3* square root of 2)

now lets remove the brackets

remember to multiply the number outside the square root; so gives us 33 =9...