Step-by-step explanation:
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The answer is 2.86 because you would divide 10 by 3.5
Use the attached figure to guide yourself through the explanation. In the figure:

.
We know that:

Also remmeber that the area of a triangle is:

The hegiht is shown in the figure in red, the base is just the b side.
Using the angle C the area of the triangle is:

Now taking the derivative of the area 2A with respect to t we have:

Replacing with the right values:

yields:
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Answer:</h3>
C. the circles are similar
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Given radius:</h3>
Circle A = 7 cm
Circle B = 21 cm
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1) Similar Shape : they can be enlarged and diminished where size varies.
2) Congruent Shape : they must have similar size and lengths.
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Hence, here the circles are similar with different lengths.