# Thread: solve for a

1. ## solve for a

Prove that cos a + cos 2a + cos 3a + cos 4a = 4 cos (a/2)cos a cos (5a/2)

i can prove this .

hence , find all the values of a between 0 and 180 such that

cos a + cos 2a + cos 3a + cos 4a =0

so i know i should start like this

4 cos (a/2)cos a cos (5a/2)=0

but i am unsure how to continue here .

2. Hi thereddevils

The next step is :

cos(a/2) = 0 or cos (a) = 0 or cos (5a/2)=0

Solve each case using general solution of trigonometry, then find the particular solution for a

3. Originally Posted by songoku
Hi thereddevils

The next step is :

cos(a/2) = 0 or cos (a) = 0 or cos (5a/2)=0

Solve each case using general solution of triginometry, then find the particular solution for a

thanks again songoku .. u r really helpful .

4. You're welcome again thereddevils