Answer:
The equivalent expression is cos(40° – 10º) ⇒ D
Step-by-step explanation:
The rule of cosine the compound angles is
cos (A + B) = cos A cos B - sin A sin B
cos (A - B) = cos A cos B + sin A sin B
Let us use these rules to solve the question
∵ cos(40°)cos(10°) + sin(40°)sin(10º)
∵ The middle sign is plus
∴ We will use the 2nd rule above
→ Compare the rule by the given expression to find the value of A and B
∵ A = 40° and B = 10°
∴ cos (A - B) = cos (40° - 10°)
∴ cos (40° - 10°) = cos(40°)cos(10°) + sin(40°)sin(10º)
∴ The equivalent expression is cos(40° – 10º)
