a = akar 7 tambah akar 5 , b = akar 7 dikurang akar 5 maka nilai ab per a ditambah b

misal x= akar 7
y=akar5
a=(×+y)
b =(x-y)

ab= (×+y)(x-y)
= x kuadrat -y kuadrat
=7-5
=2
a+b= (x+y)+(x-y )
= 2x
= 2 akar 7

jadi a*b/ (a+b)= 2/ 2 akar 7
= 1 / akar 7
= 1/7 akar 7