a) (12+6)*(7-3-1)+12 = 66
b) ((12-10)*(5+7) - 7) * 2 = 34
c) 5 - (6 + (7 - (8 + 9))) = 9
d) 5 - (4 + (3 - (2 - 1))) = -1
a) (a+b)*(c+2*d) = [a b + c 2 d * + *]
b) (2*a-3*d)*c+2*b = [2 a * 3 d * - c 2 b * *]
c) (a+b+2*c)*d = [a b + 2 c * + d *]
d) 3*a-(2*b+c)*d = [3 a * 2 b * c + d * -]
