(a) Steps: 1 Mark
     Correct answer: 1 Mark
(b) Reason: 1 Mark
     Correct answer: 1 Mark
(a)

In the figure, 1, 2 and 3 are the interior angles whereas 4, 5 and 6 are exterior angles. Using linear pair axiom, we can say that:
$\mathrm{âˆ}1+\mathrm{âˆ}4={180}^{∘}$

$\mathrm{âˆ}3+\mathrm{âˆ}6={180}^{∘}$

$\mathrm{âˆ}2+\mathrm{âˆ}3={180}^{∘}$

Adding all of them, we get:

$\mathrm{âˆ}\left(\right(1+4)+(3+6)+(2+5\left)\right)={540}^{∘}$

Rearranging the terms, we get:

$\mathrm{âˆ}\left(\right(1+2+3)+(4+5+6\left)\right)={540}^{∘}$

$\mathrm{âˆ}(1+2+3)={180}^{∘}$ (Angle Sum Property of a triangle)

So, $\mathrm{âˆ}\left(\right(4+5+6\left)\right)={540}^{∘}−{180}^{∘}={360}^{∘}$

So, the sum of the exterior angles of a triangle is ${360}^{∘}$
(b) Suppose such a triangle is possible. Then the sum of the lengths of any two sides would be greater than the length of the third side. Let's check this.
Is 4.5 + 5.8 > 10.2?   Yes
 Is 5.8 + 10.2 > 4.5?   Yes
Is 10.2 + 4.5 > 5.8?   Yes
Therefore, the triangle is possible.