1-tan^2 30derajat+tan^4 30 drjt+.....+(-1)^n tan^2n 30 drjt dit: jumlah deret..?

1 - tan^2 30 + tan^4 30 - tan^6 30 + ...

diketahui a = 1 dan r = -tan^2 30 = - (1/√3)² = -1/3
maka jumlah deret di atas adlah :
S = a/(1 - r) = 1/(1 - (-1/3)) = 1/(1 + 1/3) = 1 / (4/3) = 3/4