document.write( "Question 77922: find all solutions for equation with value of x between 0 and 360 degrees:
\n" ); document.write( "1. sin(4x) + sin x = 0
\n" ); document.write( "I have 2(2sin(x) cos (x))(cox(2x)) + sin x = 0
\n" ); document.write( "so far but don't know which identity to use for cos(2x) that will help solve.\r
\n" ); document.write( "\n" ); document.write( "This is from my daughter's textbook but I don't have the book with me now. She's taking pre-calc in high school. Thanks
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Algebra.Com's Answer #55857 by jim_thompson5910(35256)\"\" \"About 
You can put this solution on YOUR website!
\"sin%284x%29+%2B+sin+x+=+0\" Start with the given expression\r
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\n" ); document.write( "\n" ); document.write( "\"sin%282%2A%282x%29%29+%2B+sin+x+=+0\" Rewrite \"sin%284x%29\" into \"sin%282%2A%282x%29%29\"\r
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\n" ); document.write( "\n" ); document.write( "\"2sin%282x%29cos%282x%29+%2B+sin+x+=+0\" Use the identity: \"sin2x=2sinxcosx\"\r
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\n" ); document.write( "\n" ); document.write( "\"2%282sin%28x%29cos%28x%29%29cos%282x%29+%2B+sin+x+=+0\" Use the identity: \"sin2x=2sinxcosx\" again\r
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\n" ); document.write( "\n" ); document.write( "\"%284sin%28x%29cos%28x%29%29%282%28cos%28x%29%29%5E2-1%29+%2B+sin+x+=+0\" Use the identity: \"cos2x=2%28cos%28x%29%29%5E2-1\"\r
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\n" ); document.write( "\n" ); document.write( "\"8sin%28x%29%28cos%28x%29%29%5E3-4sin%28x%29cos%28x%29+%2B+sin+x+=+0\" Distribute \"4sin%28x%29cos%28x%29\"\r
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\n" ); document.write( "\n" ); document.write( "\"sin%28x%29%288%28cos%28x%29%29%5E3-4cos%28x%29+%2B+1%29+=+0\" Factor out a sin(x)\r
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\n" ); document.write( "\n" ); document.write( "Since we know the value of x for \"sin%28x%29=0\" (the solution is x=0) we can ignore the sin(x) and try to solve the expression in the parenthesis\r
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\n" ); document.write( "\n" ); document.write( "\"8%28cos%28x%29%29%5E3-4cos%28x%29+%2B+1+=+0\" So lets focus on the terms in the parenthesis\r
\n" ); document.write( "\n" ); document.write( "Let \"y=cos%28x%29\"\r
\n" ); document.write( "\n" ); document.write( "So we get\r
\n" ); document.write( "\n" ); document.write( "\"8y%5E3-4y%2B1=0\"\r
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\n" ); document.write( "\n" ); document.write( "\"8y%5E3-2y-2y%2B1=0\" Rewrite -4y into -2y-2y. This will allow us to factor\r
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\n" ); document.write( "\n" ); document.write( "\"2y%284y%5E2-1%29-%282y-1%29=0\" Group like terms and factor out the GCF\r
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\n" ); document.write( "\n" ); document.write( "\"2y%282y%2B1%29%282y-1%29-%282y-1%29=0\" Factor \"4y%5E2-1\" into \"%282y%2B1%29%282y-1%29\" using difference of squares\r
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\n" ); document.write( "\n" ); document.write( "\"%282y%282y%2B1%29-1%29%282y-1%29=0\" Combine like terms (note: the common term is \"2y-1\")\r
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\n" ); document.write( "\n" ); document.write( "Now set each factor equal to zero. Lets start with \"2y-1\"\r
\n" ); document.write( "\n" ); document.write( "\"2y-1=0\"\r
\n" ); document.write( "\n" ); document.write( "\"2y=1\"\r
\n" ); document.write( "\n" ); document.write( "\"y=1%2F2\"\r
\n" ); document.write( "\n" ); document.write( "Now let \"cos%28x%29=1%2F2\" and solve for x note: I'm using radians\r
\n" ); document.write( "\n" ); document.write( "\"x=1.04719%2Bpi%2An\" and \"x=-1.04719%2Bpi%2An\" Since our interval is [0,2pi] we must ignore the negative answer. So of these two answers, \"x=1.04719%2Bpi%2An\" is the only solution.\r
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\n" ); document.write( "\n" ); document.write( "Now let \"2y%282y%2B1%29-1=0\"\r
\n" ); document.write( "\n" ); document.write( "\"4y%5E2%2B2y-1=0\" Distribute the 2y\r
\n" ); document.write( "\n" ); document.write( "Use the quadratic formula to solve for y\r
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Solved by pluggable solver: SOLVE quadratic equation with variable
Quadratic equation \"ay%5E2%2Bby%2Bc=0\" (in our case \"4y%5E2%2B2y%2B-1+=+0\") has the following solutons:
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\n" ); document.write( " \"y%5B12%5D+=+%28b%2B-sqrt%28+b%5E2-4ac+%29%29%2F2%5Ca\"
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\n" ); document.write( " For these solutions to exist, the discriminant \"b%5E2-4ac\" should not be a negative number.
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\n" ); document.write( " First, we need to compute the discriminant \"b%5E2-4ac\": \"b%5E2-4ac=%282%29%5E2-4%2A4%2A-1=20\".
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\n" ); document.write( " Discriminant d=20 is greater than zero. That means that there are two solutions: \"+x%5B12%5D+=+%28-2%2B-sqrt%28+20+%29%29%2F2%5Ca\".
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\n" ); document.write( " \"y%5B1%5D+=+%28-%282%29%2Bsqrt%28+20+%29%29%2F2%5C4+=+0.309016994374947\"
\n" ); document.write( " \"y%5B2%5D+=+%28-%282%29-sqrt%28+20+%29%29%2F2%5C4+=+-0.809016994374947\"
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\n" ); document.write( " Quadratic expression \"4y%5E2%2B2y%2B-1\" can be factored:
\n" ); document.write( " \"4y%5E2%2B2y%2B-1+=+4%28y-0.309016994374947%29%2A%28y--0.809016994374947%29\"
\n" ); document.write( " Again, the answer is: 0.309016994374947, -0.809016994374947.\n" ); document.write( "Here's your graph:
\n" ); document.write( "\"graph%28+500%2C+500%2C+-10%2C+10%2C+-20%2C+20%2C+4%2Ax%5E2%2B2%2Ax%2B-1+%29\"

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\n" ); document.write( "\n" ); document.write( "So if we replace y with \"cos%28x%29\" we get these solutions\r
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\n" ); document.write( "\n" ); document.write( "\"cos%28x%29=0.309017\" or \"cos%28x%29=-0.809017\"\r
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\n" ); document.write( "\n" ); document.write( "Take the arccosine of both sides (for the solution \"cos%28x%29=0.309017\") to solve for x\r
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\n" ); document.write( "\n" ); document.write( "\"x=1.25664%2Bpi%2An\" or \"x=-1.25664%2Bpi%2An\" Here are 2 more possible solutions. Since our interval is [0,2pi] we must ignore the negative answer. So of these two answers, \"x=1.25664%2Bpi%2An\" is the only solution.\r
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\n" ); document.write( "\n" ); document.write( "Now lets use the other answer of \"cos%28x%29=-0.809017\"\r
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\n" ); document.write( "\n" ); document.write( "\"cos%28x%29=-0.809017\"\r
\n" ); document.write( "\n" ); document.write( "Take the arccosine of both sides\r
\n" ); document.write( "\n" ); document.write( "\"x=2.51327%2Bpi%2An\" or \"x=-2.51327%2Bpi%2An\" Since our interval is [0,2pi] we must ignore the negative answer. So of these two answers, \"x=2.51327%2Bpi%2An\" is the only solution.\r
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\n" ); document.write( "\n" ); document.write( "So after all of that, we find that our solutions are\r
\n" ); document.write( "\n" ); document.write( "\"x=0\" (from sin(x)=0) or \"x=1.04719%2Bpi%2An\" or \"x=1.25664%2Bpi%2An\" or \"x=2.51327%2Bpi%2An\"\r
\n" ); document.write( "\n" ); document.write( "As always, we can check our work by using a calculator.\r
\n" ); document.write( "\n" ); document.write( "This is a lot to take in, so feel free to ask me further about any of this.
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