1)b1=2, g=3, bn=54, n-?
bn=b1×g^n-1
2×3^n-1=54
3^n-1=27
n=4
2)a1=1, an=200,n=100, Sn-?
Sn=a1+an/2 ×n
Sn=1+200/2 ×100
Sn=10050
3)a7=21,s7=210, a1-?,d-?
s7=a1+a7/2 ×7
a1=2S7/n-a7
a1=2×210/7 -21=39
a7=a1+6d
d=a7-a1/6=21-39/6=-3
25a²-10ab+b² -1 =
=(25a²-10ab+b²) - 1² =
=(5a-b)² - 1² =
=(5a-b-1)(5a-b+1)