Question 102747
An x-intercept is a point on the x-axis where the graph of the function crosses the x-axis.
But any (x,y) point on the x-axis has zero as its y-value.  So to find the intercept
points on the x-axis, set y equal to zero and solve for the corresponding values of x. So
let's do that ... set y = 0 in the given function of {{{y = x^2 + 4x}}}. When you do that
you get:
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{{{0 = x^2 + 4x}}}
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Let's transpose this ... switch sides around ... to get it in the little more standard form
of:
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{{{x^2 + 4x = 0}}}
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Since the two terms on the left side both contain x, we can factor an x from each of the
terms to make the equation become:
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{{{x*(x + 4) = 0}}}
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This equation will be true if either one of the factors equals zero because multiplying
the left side by zero will make it equal the right side which is zero.
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So the equation will be true if either x = 0 or x + 4 = 0. In the second factor this means
that x = -4. Since we already know that y is zero for these two values, the x-intercept 
points are (0, 0) and (-4, 0).
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That's all there is to it. 
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Hope this helps you to understand that to find x-intercepts you set y equal to zero and solve
for the corresponding values of x.
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