Question

1. c 70° 21x - 1 d 8x + 7 b k

Explanation:

Step1: Use exterior angle theorem

The exterior angle theorem states that an exterior angle of a triangle is equal to the sum of the two non - adjacent interior angles. So, \(21x - 1=(8x + 7)+70\).

Step2: Simplify the equation

$$
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$$

Step3: Move the terms with \(x\) to one side

Subtract \(8x\) from both sides: \(21x-8x - 1=8x-8x + 77\), which gives \(13x-1 = 77\).

Step4: Solve for \(x\)

Add \(1\) to both sides: \(13x-1 + 1=77 + 1\), so \(13x=78\). Then divide both sides by \(13\): \(x=\frac{78}{13}=6\).

Answer:

\(x = 6\)