4 sin^2 θ + 13cos^2 θ = 7 sin^2 θ = 1 - cos^ θ 4 - 4cos^2 θ + 13cos^2 θ = 7 9cos^2 θ = 3 cos^2 θ = 1/3 cos θ = # (1/3) # - square root Square root of (1/3) has +1/3 and -1/3 as values of cos Find the key angle by doing the cos inverse of #1/3 K.A = cos^-1 #(1/3) = 0.955 θ lies in all 4 quadrants The values of θ are: θ = 0.955, 2.186, 4.096, 7.23 Ignore 0.955, 2.186, 4.096, 7.23 as they are out of range pi/2 = 1.571 The the value of θ = 0.955 = 0.96 (to 2 d.p) radian
