Question

Prove that sin^4x + cos^4x= sinxcosx

Answer 1

Answer:

(the relation you wrote is not correct, there may be something missing, so I will simplify the initial expression)

Here we have the equation:

$sin^4(x) + cos^4(x)$

We can rewrite this as:

$(sin^2(x))^2 + (cos^2(x))^2$

Now we can add and subtract cos^2(x)*sin^2(x) to get:

$(sin^2(x))^2 + (cos^2(x))^2 + 2*cos^2(x)*sin^2(x) - 2*cos^2(x)*sin^2(x)$

We can complete squares to get:

$(cos^2(x) + sin^2(x))^2 - 2*cos(x)^2*sin(x)^2$

and we know that:

cos^2(x) + sin^2(x) = 1

then:

$1 - 2*sin(x)^2*cos(x)^2$

This is the closest expression to what you wrote.

We also know that:

sin(x)*cos(x) = (1/2)*sin(2*x)

If we replace that, we get:

$1 - \frac{sin^2(2*x)}{2}$

Then the simplification is:

$cos^4(x) + sin^4(x) = 1 - \frac{sin^2(2*x)}{2}$