For which angle 0 is cos 0= -1?

Some boys and girls were asked if they had a pet. one third of the group were boys. there were 20 boys. 20% of the girls had a pet. boys with a pet : girls with a pet = 1 : 5 i need to know how many girls there are.

If cos(2x) = tan^2(y), show that cos(2y) = tan^2(x).

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