Question

Find a1, S15 = -90 and d= -7​

Answer 1

Answer:

S15=152[2a1+(15−1)(−7)]=−90

⟹152[2a1+14(−7)]=−90

⟹15[a1+7(−7)]=−90

⟹a1−49=−6

⟹a1=43