Question

Which of these statements is true? Sin 18° = cos 72°, sin 55° = cos 55°, sing 72° = cos 18°c both a and c

Answer 1

Sin 18° = cos 72° and sin 72° = cos 18°

Explanation:

According to trigonometry identity;

sin Ф = cos(90 - Ф)

for Sin 18° = cos 72

Ф = 18°

Substitute into the expression

sin Ф = cos(90 - Ф)

sin 18 = cos(90 - 18)

Sin 18 = cos 72

Hence the statement id true

For sin 72° = cos 18

Ф = 72°

Substitute into the expression

sin Ф = cos(90 - Ф)

sin 72 = cos(90 - 72)

Sin 72 = cos 18

Hence this statement too is correct

The two correct statements are Sin 18° = cos 72° and sin 72° = cos 18°