Question

9. Egalitatea a×7+15=1+2+...+12 are loc pentru valoarea lui a egala cu:9 5 6 7 8

Care este a din cele 5 numere??
Pls este tema​

Answer 1

 

$\displaystyle\bf\\a\times7+15=1+2+...+12\\\\a\times7+15=\frac{12(12+1)}{2}\\\\a\times7+15=\frac{12\times13}{2}\\\\a\times7+15=6\times13\\\\a\times7+15=78\\\\a\times7=78-15\\\\a\times7=63\\\\a=\frac{63}{7}\\\\\boxed{\bf a=9}$