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Let's assume the triangle having sides as A, B & C, With an extended exterior side as D. (Refer the above attachement for better view!)
We know that the angles opposite to equal sides of a triangle are equal.
∴ AB = AC ⇒ ∠B = ∠C
Now, Side BA of Triangle ABC has been produced to D.
∴ ∠CAD = ∠B + ∠C (Exterior angle = sum of int. opp. angles)
⇒ 3x - 10 = 25 + x + 15
⇒ 3x - 10 = 40 + x
⇒ 3x - x = 40 + 10
⇒ 2x = 50
⇒ x = 50/2
⇒ x = 25
∴ The value of 'x' in the given figure is 25!
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