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b= 3
S15= 15/2(2(2)+14(3))
= 15/2(4+52)
= 15/2(56)
=15(28)
= 420
2.
S15= 15/2((2(8)+14(2)
= 15/2(16+28)
= 15/2(2(8+14)
= 15(22)
=330
b=18-20=-2
n=15
Un=a+(n-1).b
=20+(15-1).(-2)
=20+14.(-2)
=20+(-28)
=8