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Dacă a+b=20,b+c=30 calculați 30+7b+4c=

Răspuns :

a+b=20  =>3(a+b)=60
b+c=30  =>4(b+c)=120
3a+3b+4b+4c=3a+7b+4c=60+120=180
3a+7b+4c=
a+b+a+b+a+b+ b+c+b+c+b+c+b+c=
20+20+20+30+30+30+30=
2×3+3×4=
60+120=180