1)x-suma initiala
2/5 din x= 2/5 * x= 2x/5 -mingea
x-2x/5= (5x-2x)/5= 3x/5 -restul
3/4 din 3x/5= 3/4 * 3x/5 = 9x/20 -cartea
3x/5 -9x/20= (12x-9x)/20= 3x/20 -noul rest
3x/20= 21
x=20*21/3
x=20*7
x=140
2)P=48
P=2l+2L=48
2(L+l)=48
L+l=24
L=24-l
A=L* l = l * (24-l)= 24l -l²
l+2
L-6
A=(l+2)*(L-6)= (l+2)* (24-l-6) = (l+2)*(18-l)
(l+2) * (18-l) = 24l -l² -16
18l -l² +36 -2l = 24l -l² -16
16l +36 = 24l -16
16l-24l = -16 -36
-8l = -52
8l=52
l=6,5
L=24-6,5
L= 17,5
3)ΔABC, mas<A=90
BC²=AB² +AC²
BC²= 8² + (BC-5)²
BC² =64 + BC² -10BC +25
BC²-BC² -89 = -10BC
-89= -10BC
89=10BC
BC=8,9
AC=8,9-5
AC=3,9