.I cannot proof 2 problem.....*-*!!!

1.Prove that if gcd(a,b)=1,then gcd(a+b,ab)=1.

Say :

(1)

(2)

Now multiply (1) by b: and substract (2):

Now do as above but multiplying (1) by a and substracting (2) and you'll get that also , so...

Tonio

2.If gcd(a,b)=1,and c|(a+b),then gcd(a,c)=gcd(b,c)=1.

(I can show proof gcd(a,c)=1 but I can show gcd(b,c)=1.)

....Thank !