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1. By expressing
cos3a = 2a+a , show that ,
cos3a=4cos^3a-cosa

Im really stuck
2. (Original post by Semaphore)
By expressing
cos3a = 2a+a , show that ,
cos3a=4cos^3a-cosa

Im really stuck
Try using cosine formulae.
3. (Original post by Azhar Rana)
Try using cos(a+b).
So
cos(2a+a) = 4cos^3a - cosa

What to do next?:/
4. replace cos(2α + α) with cos(2α)cos(α) – sin(2α)sin(α).
5. $Cos(A+B)=cosAcosB-sinAsinB$
Try that, however i do not want to tell you all of the information but obviously you've already done 1/3 of the work in the way you expressed cos(3x) as cos(2x+x)
Also note the formula for
$Sin(2A)=2sinAcosA$
$Cos(2A)=Cos^2(A)-Sin^2(A)$
6. (Original post by Semaphore)
So
cos(2a+a) = 4cos^3a - cosa

What to do next?:/
That's what you're supposed to be showing!

If you know the formula for cos(A + B) then put A = 2a and B = a into the formula and then rearrange using a standard identity that links cos^2 x and sin^2 x

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