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dacă
a+b=5
b+c=7
2a+5b+3c=?


Răspuns :

A+b= 5

b+c= 7

2a +5b+3c= 2(a+b) +3(b+c)

2a+5b+3c= 2 x 5 + 3 x 7

2a+ 5b+3c= 10 + 21

2a+ 5b+3c= 31

a+b = 5⏐×2  ⇔  2a+2b = 10  ①

b+c = 7⏐×3  ⇔  3b+3c = 21  ②

Din ① + ② ⇒ 2a+2b+3b+3c = 10+21

2a+5b+3c = 31