Answer: angle ITU = 145 degrees
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Step-by-step explanation:
Angle STU is the largest angle composed of the smaller angles STI and ITU.
In other words, the two smaller angles add up to the larger one.
(angle STI) + (angle ITU) = angle STU
( 20 ) + ( 12x+1 ) = 13x + 9
20+12x+1 = 13x+9
12x+21 = 13x+9
21-9 = 13x-12x
12 = x
x = 12
Using this x value, we find that,
angle ITU = 12x+1 = 12(12)+1 = 145 degrees
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As a side note,
angle STU = 13x+9 = 13(12)+9 = 165 degrees
This is confirmed with
(angle STI) + (angle ITU) = (20) + (145) = 165 = angle STU