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Question:
solve by cross multiplication method- x y = a b , ax - by = a square - b square
Answer:

Given,

x + y = a + b      ....................1

ax - by = a2 - b2   .....................2

Multiply by b in equation 1 we, get

bx + by = ab + b2      ....................3

Add equation 2 and 3, we get

       ax - by + bx + by = (a2 - b2 ) + (ab + b2 )

=> ax + bx = a2 - b2  + ab + b2

=> x(a + b) = a2 + ab

=> x(a + b) = a(a + b)

=> x = a(a + b)/(a + b)

=> x = a

From equation 3, put the value of x, we get

       ab + by = ab + b2     

=> by = ab + b2  - ab

=> by = b2

=> y = b2 /b

=> y = b

So, x = a and y = b

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