3sina + cosa = 0 find 1 / cos square a + 2sinacosa

If 3sina + cosa = 0, Tana = - 1 / 3, and 1 / (COS & sup2; a + 2sinacosa) = [Sin & sup2; a + cos & sup2; a] / [cos & sup2; a + 2sinacosa]
=[1＋tan²a]/[1＋2tana]=10/3.

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