Given: cos a = 24/25 and sin a < 0 What is cos(a + 11pi/6)?

Posted on 2015-10-07 05:40:00 in Math by JEREMY

Here's what I've done so far. What am I doing wrong?

Used Pythagorean to find sin a = 7/25

cos(a + 11pi/6) = Use addition formula

cos a * cos 11pi/6 - sin a * sin 11pi/6 = move trig functions to Quad I

cos a * cos pi/6 - sin a * - sin pi/6 = replace trig functions with known values

24/25 * sqrt(3)/2 + 7/25 * -1/2 = multiply

(24*sqrt(3))/50 + 7/50 = add like denominators

(24*sqrt(3) + 7) / 50 = Final answer marked wrong by automated homework system

Also tried splitting it into 7/50 + (12*sqrt(3))/25

If it matters, homework system is webassign.net. I insert the actual square-root symbol in the answer and format the fractions properly. Not sure how to do either of those in yahoo answers.

---

Answer Comment

No Answers Posted Yet.