an=4n+3
a1=4*1+3=7
a30=4*30+3=123
S30=(a1+a30)*30/2=(7+123)*15=130*15=1950
S25-S15=(a1+a25)*25/2-(a1+a15)*15/2=(a1+a1+(25-1)d)*25/2-(a1+a1+(15-1)d)*15/2=(2a1+24d)*25/2-(2a1+14d)*15/2=(a1+12d)*25-(a1+7d)*15=5[5(a1=12d)-3(a1+7d)]=5[5(7+2.5*12)-3(7+2.5*7)]=5[5(7+15)-3*7(1+2.5)]=5[5*22-21*3.5]=5[110-73.5]=5*36.5=182.5
an=4n+3
a1=4*1+3=7
a30=4*30+3=123
S30=(a1+a30)*30/2=(7+123)*15=130*15=1950
S25-S15=(a1+a25)*25/2-(a1+a15)*15/2=(a1+a1+(25-1)d)*25/2-(a1+a1+(15-1)d)*15/2=(2a1+24d)*25/2-(2a1+14d)*15/2=(a1+12d)*25-(a1+7d)*15=5[5(a1=12d)-3(a1+7d)]=5[5(7+2.5*12)-3(7+2.5*7)]=5[5(7+15)-3*7(1+2.5)]=5[5*22-21*3.5]=5[110-73.5]=5*36.5=182.5