Belajar matematika sejak dini
71.(SPMB '05)Jika3log2=pdan2log7=q,maka14log54=....a.p+3p+qb.2pp+qc.p+3p(q+1)d.p+qp(q+1)e.p(q+1)p+qJawab:c14log54=...log54...log14=...log(2.27)...log(2.7),pilih basis 2mengapa tidak pilih basis selain 2lihat penyebut, di sana terdapat numerus 7pada soal, pasangan numerus 7 adalah 2,makanya basis 2 dipilih, bukan yang lain=2log(2.27)2log(2.7)=2log2+2log272log2+2log7=2log2+2log332log2+2log7=2log2+(3×13log2)2log2+2log7=1+3p1+q=p+3p(q+1).
72.(SPMB '04)Jikaa>1,maka penyelesaian untuk(alog(2x+1))(3loga)=1adalah.... a.1b.2c.3d.4e.5Jawab:d(alog(2x+1))(3loga)=1(3loga)(alog(2x+1))=1(3loga.12)(alog(2x+1))=112(3loga)(alog(2x+1))=13log(2x+1)=22x+1=322x=9−12x=8x=4.
73.(SPMB '04)Nilai(5log10)2−(5log2)25log20adalah.... a.12b.1c.2d.4e.5Jawab:c(5log10)2−(5log2)25log20=(5log10+5log2)(5log10−5log2)5log(20).12=5log(10.2)×5log(102)12×5log20=2×(5log205log20)×5log5=2.1.1=2.
74.(SPMB '03)Jika diketahui bahwa4log4logx−4log4log4log16=2maka....a.2logx=8b.2logx=4c.4logx=8d.4logx=16e.16logx=8Jawab:c4log4logx−4log4log4log16=2⇔4log4logx−4log4log4log42=2⇔4log4logx−4log4log2=2⇔4log4logx−4log22log21=2⇔4log4logx−4log(12)=2⇔4log4logx−22log2−1=2⇔4log4logx−(−12)=2⇔4log4logx+12=2⇔4log4logx=2−12=32⇔4logx=4.32⇔4logx=(22).32⇔4logx=23⇔4logx=8.
75.(UMPTN '92)Jikaxmemenuhi persamaan4log4logx−4log4log4log16=2maka nilai16logx=....a.4b.2c.1d.−2e.−4Jawab:a4log4logx−4log4log4log16=2menyebabkan4logx=8⇒x=48(lihat pembahasan no.23)maka,16logx=16log48=42log48=82=4.
78.(UMPTN '94)Hasil kali akar-akar persamaan3logx.(2+3logx)=15adalah....a.19b.13c.1d.3e.9Jawab:a3logx.(2+3logx)=15⇔(2+3logx)3logx−15=0⇔23logx+(3logx)2−15=0⇔(3logx)2+23logx−15=0⇔(3logx1+5)(3logx2−3)=0⇔3logx1+5=0atau3logx1−3=0⇔3logx1=−5atau3logx2=3⇔x1=3−5ataux2=33maka⇔x1×x2=3−5×33=3−5+3=3−2⇔=132⇔=19.
79.Diketahui bahwa2log3=pdan3log11=q,maka nilai44log66=....Jawab:44log66=...log66...log44=...log(2×3×11)...log(22×11)=3log2+3log3+3log113log22+3log11=12log3+3log3+3log1123log22+3log11=1p+1+q2p+q=1p+1+q2p+q×pp=1+p+pq2+pq
80.(AIME 1984)Diketahui bahwaxdanyadalah bilangan real yang memenuhi{8logx+4logy2=58logy+4logx2=7Tentukanlah nilai darixyJawab:8logx+4logy2=5⇔23logx+22logy2=5....(1)8logy+4logx2=7⇔23logy+22logx2=7....(2)selanjutnya,13.2logx+2logy=5|×13|⇒19.2logx+13.2logy=53....(3)2logx+13.2logy=7|×1|⇒2logx+13.2logy=7....(4)saat persamaan(3)−(4)=−89.2logx=53−7=−163maka2logx=(−163)(−98)2logx=6⇔x=26⇔x=64Pada persamaan 1 selanjutnya13.2logx+2logy=5⇔13.2log26+2logy=5⇔13.6+2logy=5⇔2+2logy=5⇔2logy=5−2=3⇔y=23=8Jadi,x.y=64.8=512
81.Tentukanlah nilai daria.(22log6)(39log5)(515log2)b.(27log125)(25log164)(64log19)c.(625log19)(7log125)(19log7)Jawab:a.(22log6)(39log5)(515log2)=(22log6)(332log5)(55−1log2)=(22log6)(33log512)(55log2−1)=(22log6)(33log5)(55log12)=6×5×12=35b.(27log125)(25log164)(64log19)=(33log53)(52log4−3)(43log3−2)=33.(−32).(−23).3log5.5log4.4log3=1Pembahasan diserahkan kepadaPembaca yang budiman untuk poin c
82.Tentukanlah nilaia+bdimanaadanbadalah bilangan riil positif.7log(1+a2)−7log25=7log(2ab−15)−7log(25+b2)Jawab:7log(1+a2)25=7log(2ab−15)(25+b2)diambilpersamaannya, maka(1+a2)25=(2ab−15)(25+b2)(1+a2)(25+b2)=25(2ab−15){(1+a2) faktor dari25,a>0,a∈Ratau(25+b2) faktor dari 25,b>0,b∈RjugaNoa(1+a2)b(25+b2)Keterangana+b12510125Memenuhi122750⋯⋯tidak⋯3⋯⋯550tidak⋯
83.Jika60a=3dan60b=5maka hasil dari12(1−x−y2(1−b))Jawab:Perhatikanlah bahwa{60a=3⇒60log3=a60b=5⇒60log5=bSelanjutnyaUntuk(1−a−b),maka1−a−b=1−60log3−60log5=60log60−60log3−60log5=60log603×5=60log4=60log22Untuk2(1−b),maka2(1−b)=2(1−60log5)=2(60log60−60log5)=2(60log605)=2(60log12)=60log122Untuk(1−x−y2(1−b)),maka(1−x−y2(1−b))=60log2260log122=260log2260log12=60log260log12=12log2Jadi,12(1−x−y2(1−b))=1212log2=2
84.Diberikan bilangan riil positifx,y,danzyang memenuhi persamaan2xlog(2y)=22xlog(4z)=24xlog(8yz)≠0.Jika nilaixy5zdapat dinyatakan dengan12pqdenganpdanqbilangan asli yang saling prima,maka nilai darip+q=....Jawab:2xlog(2y)=22xlog(4z)=24xlog(8yz)≠0makaxlog(2y)=2xlog(4y)⇒log(2y)×log(2x)=logx×log(4y)...(1)xlog(2y)=4xlog(8yz)⇒log(2y)×log(4x)=logx×log(8yz)....(2)2xlog(4y)=4xlog(8yz)⇒log(4y)×log(4x)=log(2x)×log(8yz)....(3)Perhatikan persamaan(2),yaitu:log(2y)×log(4x)=logx×log(8yz)log(2y)×(log(2x)+log2)=logx×log(8yz)log(2y)×log(2x)+log(2y)×log2=logx×log(8yz)logx×log(4y)+log(2y)×log2=logx×log(8yz)persamaan di atas, persamaan(1)disubstitusikanlog(2y)=logx×log(8yz)−logx×log(4y)log2log(2y)=logx×(log8yz4y)log2log(2y)=logx×log(2z)log2......(4)Perhatikan juga persamaan(3),yaitu:log(4y)×log(4x)=log(2x)×log(8yz)(log(2y)+log2)×log(4x)=log(2x)×log(8yz)log(2y)×log(4x)+log2×log(4x)=log(2x)×log(8yz)logx×log(8yz)+log2×log(4x)=log(2x)×log(8yz)di atas, persamaan(2)disubstitusikanlog2×log(4x)=log(2x)×log(8yz)−logx×log(8yz)log2×log(4x)=log(8yz)×(log(2x)−logx)log2×log(4x)=log(8yz)×(log2xx)log2×log(4x)=log(8yz)×log2log4x=log(8yz)4x=8yzxz=2y....(5)
.dari persamaan(4)dan(5)log(2y)=logx×log(2z)log2log(xz)=logx×log(2z)log2log2(logx−logz)=logx×log(2z)log2×logx−log2×logz=logx×(log2+logz)log2×logx−log2×logz=logx×log2+logx×logz−log2×logz=logx×logzlog2−1=logx12=x.....(6)
.persamaan(2)Menentukan nilaizlog2y×log(4x)=logx×log(8yz)log2y×log(4(2yz))=logx×log(8yz)log2y×log(8yz)=logx×log(8yz)log(2y)=logx2y=xy=12x=12×12=14.....(7)x=2yz12=2(14)z1=z
.maka nilai untukxy5zadalahxy5z=(12).(14)5.1=12×45=12×(22)5=121+10=1211=12111=12pq{p=11q=1dan jelas bahwapdanqsaling primaJadi,p+q=11+1=12
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