What two common identities are most useful for proving that 2cos^2x = cos2x+1
I went off starting that cos2x is an identity but stuff wasn't working out...
One thing about these identities is that there may be more than one way of proving them
So...let's try this....
2cos^2 (x) = cos(2x) +1
2( 1 - sin^2 (x)) = cos (x + x) + 1
2 - 2sin^2 (x) = cos(x)cos(x) - sin(x)sin(x) + 1 subtract 1 from both sides
1 - 2sin^2(x) = cos^2(x) - sin^2(x) add 2 sin^2x to both sides
1 = cos^2(x) + sin^2(x) which is true