Question 531041
Let u = x^2.
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u^2 +8u +15 = 0
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factor
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(u+3)(u+5)=0
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substitute u = x^2
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(x^2 + 3)(x^2 + 5) = 0
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So x^2 = -3 or -5.  That means there are no real roots.
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{{{ graph(500,500,-10,10,-10,100,x^4+8*x^2+15) }}}