chia hết

C

cry_with_me

Nếu $5a^2 + 15ab - b^2 \vdots 49$

$\rightarrow 5a^2 + 15ab - b^2 \vdots 7$

$\rightarrow 30a^2 + 90ab - 6b^2 \vdots 7$

$\rightarrow 3a + b \vdots 7$

Nếu $ 3a + b \vdots 7$

$\rightarrow 3a + b = 7c$

$\rightarrow b = 7c - 3a$

$\rightarrow b^2 + 15ab - b^2 = 5a^2 + 15a(7c - 3a) - (7c-3a)^2$

$= 5a^2 + 105ac - 45a^2 - 49c^2 + 42ac - 9a^2$

$=49(-a^2 + 3ac - c^2) \vdots 49$


$\rightarrow$ đpcm
 
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