SOLUTION: Please help me to solve this assignment,Given that CosA=2/5,find the exact value of Cos2A

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Question 1190008: Please help me to solve this assignment,Given that CosA=2/5,find the exact value of Cos2A
Found 2 solutions by Alan3354, ikleyn:
Answer by Alan3354(69443) About Me  (Show Source):
You can put this solution on YOUR website!
Look up "Double angles" on Wikipedia.

Answer by ikleyn(52831) About Me  (Show Source):
You can put this solution on YOUR website!
.

Use the basic  Trigonometry formula   cos(2A) = 2*cos^2(A) - 1.

Substitute  cos(A) = 2/5  here  (as it is given),  and complete the calculation on your own.

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See the compendium of basic Trigonometry formulas in the lesson
    - FORMULAS FOR TRIGONOMETRIC FUNCTIONS
in this site.

In the future,  consult this lesson every time as you need some basic Trigonometry formula.