bonjour
3/4÷1/8-9/4÷3/14
on commence par calculer petit à petit
3/4 ÷ 1/8 = 3/4 * 8 /1 = (8*3)/4 = 24/4 = 6
on calcule maintenant
-9/4÷3/14 = -9/4 * 14 /3 = (-3*3)/(2*2) * (7*2)/3 = -3/2 * 7 = -21/2
on additionne 6 et -21/2
donc
6 + ( -21/2) = 12/2 - 21/2 = (12-21)/2 = - 9/2
bonne soirée