Nyatakan fungsi-fungsi berikut ini ke dalam bentuk kosinus:
(a) $4 \sin(\omega t – 30^\circ)$
(b) $-2 \sin 6t$
(c) $-10 \sin(\omega t + 20^\circ)$
Identitas utama yang kita gunakan adalah:
$$\sin(x) = \cos(x – 90^\circ)$$
$$-\sin(x) = \cos(x + 90^\circ)$$
(a) $4 \sin(\omega t – 30^\circ)$
Gunakan identitas $\sin(x) = \cos(x – 90^\circ)$:
$$4 \cos(\omega t – 30^\circ – 90^\circ)$$
$$\mathbf{4 \cos(\omega t – 120^\circ)}$$
(b) $-2 \sin 6t$
Gunakan identitas $-\sin(x) = \cos(x + 90^\circ)$:
$$-(-2) \cos(6t + 90^\circ)$$
$$\mathbf{2 \cos(6t + 90^\circ)}$$
(c) $-10 \sin(\omega t + 20^\circ)$
Gunakan identitas $-\sin(x) = \cos(x + 90^\circ)$:
$$ -(-10) \cos(\omega t + 20^\circ + 90^\circ)$$
$$\mathbf{10 \cos(\omega t + 110^\circ)}$$
