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Kelas 8 Matematika
Bab Kesebangunan

PR + RT = PT
RT = PT - PR
RT = 20 - PR

ST/PQ = RT/PR
4/12 = (20 - PR)/PR
1/3 = (20 - PR)/PR
PR = 3 . (20 - PR)
PR = 60 - 3 PR
PR + 3 PR = 60
4 PR = 60
PR = 60/4
PR = 15 cm