If the sum of the first n terms of an AP is 5n−n2, the common difference is ______
-2
-1
1
2
Sn=5n−n2
T1=S1=5(1)−(1)2=5−1=4
S2=T1+T2=5(2)−(2)2=10−4=6
T2=S2−T1=6−4=2
d=T2−T1=2
The sum of the first n terms of an AP whose first term is 8 and the common difference is 20 is equal to the sum of the first 2n terms of another AP, whose first term is - 30 and the common difference is 8. find n?