Answer:
a = 8
b = 4√5
Explanation:
Corresponding sides of similar triangles are proportional.
short side/long side = a/16 = 4/a
a² = 64 . . . . . . multiply by 16a
a = 8 . . . . . . take the square root
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hypotenuse/short side = b/4 = 20/b
b² = 80 . . . . . . . multiply by 4b
b = 4√5 . . . . . take the square root
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Additional comment
These relations for the right triangle can be summarized by the rule "the length of a segment that touches the hypotenuse is the geometric mean of the two hypotenuse segments it touches." In the case of b, that means ...
b = √((4)(4+16)) = 4√5
The same sort of relation applies to the unmarked vertical segment on the left:
c = √((16)(16+4)) = 8√5
And, we saw ...
a = √((4)(16)) = 8
Of course, if this rule is to make sense, you need to know that the geometric mean of two numbers is the square root of their product. (In general, the geometric mean of n numbers is the n-th root of their product.)