Question

find the value of x³+y³+15xy-125 if x+y=5

Answer 1

x cube +y cube =125 then -125 will cancelled then value of 15xy =x=5-y then 15xy=(5-y)y=5y-y square

Answer 2

answwer is 5
$(x+ y)( {x}^{2} + {y}^{2} - xy) + 15xy = 125 \\ (5)( {x}^{2} + {y}^{2} - xy) + 15xy = 125 \\ {5x}^{2} + {5y}^{2} - 5xy + 15xy = 125 \\ {5x}^{2} + {5y}^{2} + 10xy = 125 \\ 5( {x}^{2} + {y}^{2} + 2xy) = 125 \\ {x}^{2} + {y}^{2} + 2xy = 125 \div 5 \\ (x + y) { }^{2} = 25 \\ (x + y) = \sqrt{25 } \\ (x + y) = 5 \\ 5 = 5$