sn=a 1*(q^n- 1)/(q- 1)
s5=a 1*(q^5- 1)/(q- 1)= 10
s 10=a 1*(q^ 10- 1)/(q- 1)=50
Two types of division:
q^5+ 1=5
q^5=4
a 1*(q^5- 1)/(q- 1)= 10
a 1/(q- 1)= 10/3
s 15=a 1*(q^ 15- 1)/(q- 1)
=a 1*(4^3- 1)/(q- 1)
=63a 1/(q- 1)
=63*( 10/3)
=2 10