a14=35 d=3
a14=a1+d*13
35=a1+13*3
a1=35-39 = -4
a6=a1+d*5 = -4+5*3= -4+15=11
S=(a1+a6)/2* 6 =(-4+11)/2 *6= 21
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a14=35 d=3
a14=a1+d*13
35=a1+13*3
a1=35-39 = -4
a6=a1+d*5 = -4+5*3= -4+15=11
S=(a1+a6)/2* 6 =(-4+11)/2 *6= 21