# Prove the Derivative of Cosine (Taylor Series)

This example finds of the derivative of the cosine function using its taylor series definition.

## Steps

2. Take the derivative of both sides of the equation.

3. From the summation property of derivatives we can apply the power rule to each of the expressions on the right-hand side of the equation.

Simplify the factorial operator in the denominator.

Now the right-hand side is very close-to, but not quire the taylor series of the sine function[2]; each term’s sign in the expression is flipped. To fix this we can factor from each of the expressions.

Substitute in for its taylor series.

The derivative of is .

## References

1. Derive Cosine Function (Taylor Series)
Wumbo (internal)
2. Derive Sine Function (Taylor Series)
Wumbo (internal)