Question 995635
Sn=(t1+tn)*(n/2)
945=(5+tn)*(n/2)
945*2=(5+tn)*n
1890=(5+tn)*n  .......... 1

tn=t1+(n-1)d
tn=5+(n-1)4
tn=5+4n-4
tn=1+4n  ................2

substituting the value of tn in eq 1
1890=(5+1+4n)*n
1890=(6+4n)*n
1890=6n+4n^2 or writing in the standard form
4n^2+6n-1890=0
*[invoke quadratic "n", 2, 3, -945 ]
The series have 21 terms because n cannot be a fraction