a1=59
a2=55
d=a2-a1=55-59=-4
An=a1+d(n-1)
An=59-4(n-1)=59-4n+4=63-4n
63-4n>0
-4n>-63
n<15.75
a15=a1+14d
a15=59-4*14=3
S15=(a1+a15)n/2
S15=(59+3)*15/2=465
a1=59
a2=55
d=a2-a1=55-59=-4
An=a1+d(n-1)
An=59-4(n-1)=59-4n+4=63-4n
63-4n>0
-4n>-63
n<15.75
a15=a1+14d
a15=59-4*14=3
S15=(a1+a15)n/2
S15=(59+3)*15/2=465