posted by aaa .
2cos^2 theta + 5 sin theta - 4 = 0
2(1-sin^2) + 5sin - 4 = 0
2sin^2 - 5sin + 2 = 0
(2sin-1)(sin-2) = 0
sin = 1/2 or 2
can give me a further and detail explanation where did the theta on the 5 and the formula on changing the cos theta.
you know that cos^2 = 1-sin^2
the rest is just algebra I
I dropped the thetas just for ease of typing.
Naturally, sin can never be 2, so that's a non-solution, even though it does satisfy the original equation.