sec^2x+2sec=0. solve the equation for 0
u = sec x ( substitution )u² + 2 u = 0u ( u + 2 ) = 0u = 0, or u = - 21/cos x = 0 1/cos x = - 2(not accepted ) cos x = -1/2 x 1 = 2π/3, x 2 = 4π/3
