Answer:
The value of n is 21.
Step-by-step explanation:
We are given that three of the exterior angle of the n-sided polygon are 50° each two of its interior angle are 127° and 135° and the remaining interior angle are 173 each.
As we know that the sum of all exterior angles of the polygon is 360°. Also, the number of remaining interior angles will be (n - 5).
And, the exterior angle = 180° - the interior angle.
So, according to the question;

150 + 53 + 45 + 7(n - 5) = 360
248 + 7n - 35 = 360
213 + 7n = 360
7n = 360 - 213
7n = 147
n = 
n = 21
Hence, the value of n is 21 and this is a 21-sided polygon.
Answer:
Looks like <u>inverse property of addition </u>to me
An = -2n(n + 3)
a(15) = -2(15)(15 + 3) = -30(18) = -540
200 can be rounded down to 200, which is easy to add with. Thus, an estimate is 222+200=422.
Alternatively, you could round 222 up a bit, to get 225+200=425. This is actually the correct answer, but this rounding doesn't make the calculation easier, so if you needed a quick estimate I'd go with the first option.