Question

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

Answer 1

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