Solving Basic Equations
This pages shows you how to solve basic equations. It is important to write down all of the solutions in the interval that you are given.
Example
Solve sin(x - p/2) = ½ for 0 < x < 2p
We take arcsin [arcsin means sin-1] of both sides to get:
x - p/2 = arcsin(½)
x - p/2 = -7p/6 , p/6, 5p/6, 13p/6 ...
We want all of the solutions for x between 0 and 2p. You must be careful, because when you take p/2 to the right hand side, the solutions are each going to have increased by p/2 and so some solutions might have entered or exited the range that you want them to be in.
x = 2p/3, 4p/3
Example
Solve 2cos2x + 3sinx = 3, giving your answer in radians for 0< x <p.
2cos2x + 3sinx - 3 = 0
We need to get everything in terms of sinx or everything in terms of cosx. Since we know that cos2x = 1 - sin2x:
2(1 - sin2x) + 3sinx - 3 = 0
2 - 2sin2x + 3sinx - 3 = 0
-2sin2x + 3sinx - 1 = 0
2sin2x - 3sinx + 1 = 0
(2sinx - 1)(sinx - 1) = 0
sin x = ½ or sin x = 1
x = p/6, 5p/6, p/2
Remember, if sinx = 1 then x = p/2 or 5p/2 or 9p/2 or ... and similarly for arcsin(½). In this question, you are asked for values of x between 0 and p. You must write down all of the appropriate solutions.
