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Find the value of ab, if a + b = 5 and a²-b² = 75
Question

Find the value of ab, if a + b = 5 and a²-b² = 75

A.

–50

B.

10

C.

50

D.

25

Correct option is A

Given:

a + b = 5 and a²-b² = 75  

Solution: 

a + b = 5   ........equation(1)

a²-b² = 75   

(a-b)(a-b)=75  

5(a-b) = 75 

a-b = 15     ..........  equation(2)  

Add equation (1) and (2) 

a + b + a - b = 5 + 15 

a = 10 

From equation (1)

a + b = 5 

10 + b = 5 

b = -5 

then the value of ab = (-5)(10) = - 50 



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