Question

Differentiate y = sin^2x - cos^2x

Answer 1

Using the power and chain rules, the derivative of

y = sin²(x) - cos²(x)

is

dy/dx = 2 sin(x) cos(x) - 2 cos(x) (-sin(x))

dy/dx = 2 sin(x) cos(x) + 2 sin(x) cos(x)

dy/dx = 4 sin(x) cos(x)

and using the double angle identity for sine, this is equivalent to

dy/dx = 2 sin(2x)

Alternatively, we can first simplify y using the double angle identity for cosine:

cos(2x)= cos²(x) - sin²(x)

so that

y = -cos(2x)

and the derivative is

dy/dx = sin(2x) • 2

dy/dx = 2 sin(2x)