Question

Ok, so how is cos(2x)=2cos²x-1 ?

Answer 1

cos(a+b)=cosa cosb - sina sinb

if a=b=x

cos(x+x)=cos(2x)=cos^2(x) - sin^2(x)

can you think of a way to write sin^2 in terms of cos^2?

Answer 2

well sin²x=1-cos²x right?

Answer 3

yep, so replace that into the double angle formula and you should be able to simplify it to your original post

Answer 4

cos(2x)=cos(x+x)=cos(x)cos(x)-sin(x)sin(x)=cos²(x)-sin²(x)=cos²(x)-cos²(x)-1=-1 ??

Answer 5

c^2-s^2

c^2-(1-c^2)

c^2 -1 + c^2

2c^2 -1

Answer 6

i spose we could have gone with what does -s^2 equal :)

Answer 7

*sigh*, it's been so long since I needed to do any maths... thanks

Answer 8

itll be fine, i was outta school for 20+ years before going back for a degree