How do you prove $2cos^2 x - sinx - 1 = 0$ ?

Question

2193 views around the world

You can reuse this answer
Creative Commons License

Answer 1 · 2015-04-15T16:03:57

You can't prove it. It's not true.

Let $x= pi/4$ then we get:

$2 cos^2x - sinx -1 = 2 cos^2 (pi/4) - sin (pi/4) -1$

$color(white)"sssssssssssssssssss"$ $= 2 (1/sqrt2)^2- sqrt2/2-1$

$color(white)"sssssssssssssssssss"$ $= 1- sqrt2/2-1$

$color(white)"sssssssssssssssssss"$ $= - sqrt2/2$

How do you prove 2cos^2 x - sinx - 1 = 02cos^2 x - sinx - 1 = 0?