Question

if in an AP first term is 1,last term as 30 and the sum of all terms is 399,then the number of terms is ___
a)37
b)38
c)39
d)40

I need solution
pls answer fast as possible ​

Answer 1

Step-by-step explanation:

Wind is the natural movement of air or other gases relative to a planet's surface. Wind occurs on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hours, to global winds resulting from the difference in absorption of solar energy between the climate zones on Earth. The two main causes of large-scale atmospheric circulation are the differential heating between the equator and the poles, and the rotation of the planet (Coriolis effect). Within the tropics and subtropics, thermal low circulations over terrain and high plateaus can drive monsoon circulations. In coastal areas the sea breeze/land breeze cycle can define local winds; in areas that have variable terrain, mountain and valley breezes can prevail.

Answer 2

APPROPRIATE QUESTION

IF in an AP first term is 1,last term as 20 and the sum of all terms is 399,then the number of terms is ___

a)37

b)38

c)39

d)40

Given:

First term (a)=1

Last term (tn )=20

Sum of terms (Sn)=399

We know that,

tn=a+(n−1)d

Sn =n/2(2a+(n−1)d)

Sn= n/2(a+(a+(n−1)d))

⇒Sn=n/2(a+tn)

⇒399= n/2(1+20)

⇒399= 21n/2

⇒21n=399×2

⇒n=798/21

⇒n=38