SOLUTION: Find all solutions for the following equations.Record your answer in exact form where possible. (a) {{{2}}}{{{"cos"^2x}}}{{{""=""}}}{{{-3}}}{{{"sin"}}}{{{x}}} (b) {{{"sin"}}}{{{x

Algebra ->  Trigonometry-basics -> SOLUTION: Find all solutions for the following equations.Record your answer in exact form where possible. (a) {{{2}}}{{{"cos"^2x}}}{{{""=""}}}{{{-3}}}{{{"sin"}}}{{{x}}} (b) {{{"sin"}}}{{{x      Log On


   



Question 325522: Find all solutions for the following equations.Record your answer in exact form where possible.
(a) 2%22cos%22%5E2x%22%22=%22%22-3%22sin%22x
(b) %22sin%22x%22%22%2A%22%22%22tan%22x%22%22=%22%22%22sin%22x

Thinking I may have to us Half-angle formula on these (not sure)

Answer by Edwin McCravy(20060) About Me  (Show Source):
You can put this solution on YOUR website!
Find all solutions for the following equations.Record your answer in exact form where possible.
(a) 2%22cos%22%5E2x%22%22=%22%22-3%22sin%22x
(b) %22sin%22x%22%22%2A%22%22%22tan%22x%22%22=%22%22%22sin%22x

Thinking I may have to us Half-angle formula on these (not sure)

No you don't need those.

2%22cos%22%5E2x%22%22=%22%22-3%22sin%22x

Use the identity:  %22sin%22%5E2theta%22%22%2B%22%22%22cos%22%5E2theta%22%22=%22%221 written as %22cos%22%5E2theta%22%22=%22%221%22%22-%22%22%22sin%22%5E2theta to rewrite the left side:

2%281-%22sin%22%5E2x%29%22%22=%22%22-3%22sin%22x

2%22%22-%22%222%22sin%22%5E2x%22%22=%22%22-3%22sin%22x

Get 0 on the right:

2%22%22-%22%222%22sin%22%5E2x%22%22%2B%22%223%22sin%22x%22%22=%22%220

Rearrange terms on the left in descending order:

-2%22sin%22%5E2x%22%22%2B%22%223%22sin%22x%22%22%2B%22%222%22%22=%22%220

To make factoring easier, multiply through by -1

2%22sin%22%5E2x%22%22-%22%223%22sin%22x%22%22-%22%222%22%22=%22%220

%22%28sin%22x%22%22-%22%22%222%29%28%222%22sin%22x%22%22%2B%22%22%221%29%22%22%22=%22%220

Setting the first factor = 0

%22sin%22x%22%22-%22%222%22%22=%22%220

%22sin%22x%22%22=%22%222

This is impossible because no sine can be greater than 1 or
less than -1.

Setting the second factor = 0

2%22sin%22x%22%22%2B%22%22%221%22%22%22=%22%220

2%22sin%22x%22%22=%22%22%22-1%22

%22sin%22x%22%22=%22%22-1%2F2

All angles in QIII and QIV  with reference angle 30° (or pi%2F6
if your teacher wants radians) are solutions.

In degrees:
x%22%22=%22%22%22210%B0%2Bn%2A360%B0%22; x%22%22=%22%22%22330%B0%2Bn%2A360%B0%22

In radians:
x%22%22=%22%227pi%2F6%2B2pi%2An;x%22%22=%22%2211pi%2F6%2B2pi%2An
 
--------------------------------
--------------------------------
--------------------------------

%22sin%22x%22%22%2A%22%22%22tan%22x%22%22=%22%22%22sin%22x

Get 0 on the right:

%22sin%22x%22%22%2A%22%22%22tan%22x%22%22-%22%22%22sin%22x%22%22=%22%220

Factor out %22sin%22x on the left side:

%22sin%22x%22%28tan%22x%22%22-%22%22%221%29%22%22%22=%22%220

Setting the first factor = 0

%22sin%22x%22%22=%22%220

All angles with reference angle 0 are solutions:

In degrees:
x%22%22=%22%22%22n%2A180%B0%22

In radians:
x%22%22=%22%22n%2Api;

Setting the second factor = 0

%22tan%22x%22%22-%22%22%221%22%22%22=%22%220

%22tan%22x%22%22=%22%22%221%22

All angles in QI and QIII  with reference angle 45° (or pi%2F4
if your teacher wants radians) are solutions.

In degrees:
x%22%22=%22%22%2245%B0%2Bn%2A360%B0%22; x%22%22=%22%22%22225%B0%2Bn%2A360%B0%22

In radians:
x%22%22=%22%22pi%2F4%2B2pi%2An;x%22%22=%22%227pi%2F4%2B2pi%2An

Edwin