Question

My friend needed help with this and I found the height through the theorem but can't figure out how to get the base of the overall triangle

Answer 1

You found CD from the Pythagorean theorem to be ...

... CD = √(5² -2²) = √21

Since triangle ADC ~ triangle ACB, the ratios of corresponding sides are the same:

... AC/AD = AB/AC

... AB = AC²/AD

... AB = 5²/2 = 12.5 . . . . . . . the base of the overall triangle

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Then the area (A) is ...

... A = (1/2)bh

... A = (1/2)(12.5)(√21) ≈ 28.64 square units

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As you see here, the altitude of a right triangle divides it into three similar triangles. From smallest to largest, they are ...

... ADC ~ CDB ~ ACB

You can figure this using AAA similarity, since the smallest and largest triangles listed above share an acute angle vertex (∠A). That, together with the right angle, means all angles are congruent. After that, then you know ∠ACD ≅ ∠CBD, so you can show the middle sized triangle is similar to the other two.