Question

If for and AP Sn=840,a=2,tn=41 then find n = ??​

Answer 1

Sₙ=840

a=2

aₙ=41

aₙ=[a +( n-1 )d ]

n =?

Sₙ= n/2 [2a + (n-1)d] = 840

=>   n/2[ a+a+(n-1)d ] = 840

=>   n/2(a+aₙ)=840

=>   n/2(2+41)=840

=>   n/2*43=840

=>   n/2=840/43

=>   n=1680/43

=>   n=36.06