Jawabanmu

2014-09-11T19:19:42+07:00
A=6  .. b=3 ... Sn=756 ... n=???

Sn=n/2(a+Un)

Un=a+(n-1)b = 6 +(n-1)3 = 3+3n

756 = n/2( 6 + 3 +3 n)

1512 = 3n² +9n ...... n² + 3 n -  504 =0 ... (n-21)(n+24)=0 ... n=21