Recall that Sn = n/2 (2a+(n-1)d)
So, for the first one
S10 = 5(2a+9*4) = -20
2a+36 = -4
a = -20
Do the others the same way
Find rhe first term of the ff:
S sub 10= -20
d=4
S sub 15=-195
d=3
S sub 12= 378
d= 9
1 answer