which of the folllowing expressions is equivilent to cos7pi3

which of the folllowing expressions is equivilent to cos(7pi/3)

1. cos(pi/3)

2. cos(4pi/3)

3. cos(-pi/3)

4. -cos(-7pi/3)

a. 1and3

b. 3 and 4

c. 1 and 4

d. 1 and 2

cosine repeat every 360 i.e. 2pi

so,

cos(pi/3)=cos(pi/3+2pi)=cos(7pi/3)

also cos(-pi/3)=cos(pi/3)

so, a.) 1 and 3