Question

Write Cos 20° in terms of sin.?

I don't get it, i tried watching videos but still gets me nowhere. pls help

Answer 1

Sine and cosine are cofunctions. Therefore, the relationship `sinx = cos(90 - x)` and `cosx = sin(90 - x)` applies.
`cos(20) = sin(90 - 20) = sin(70)`

Explanation:

Cofunctions are pairs of trigonometric functions with the simple relationship, where, for example, `sin(alpha) = cos(beta)` where `alpha + beta = 90`. This also holds true for `csc` and `sec`, and `cot` and `tan`.

This means that any `cos(alpha)` is always equal to `sin(90 - alpha)`, or vice versa.