The length of the altitude AD is 12 units.
How to find the height of a triangle?
The height of the triangle can be found as follows:
We have to find an angle using cosine law before we can find the height.
Therefore,
20² = 13² + 21² - 2 × 21 × 13 cos B
400 = 169 + 441 - 546 cos B
400 - 610 = - 546 cos B
-210 = - 546 cos B
cos B = -210 / -546
cos B = 0.38461538461
B = cos⁻¹ 0.38461538461
B = 67.3810899783
B = 67.38°
Hence,
sin 67.38° = opposite / hypotenuse
sin 67.38° = AD / 13
cross multiply
AD = 13 sin 67.38°
AD = 11.9999882145
AD = 12 units
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