Question

if a + b = 7, ab= 15 therefore a3 - b3

Answer 1

a+b=7
(a+b)²= 7²
a²+ b²+ 2ab= 49 ....(1)
subtract -2ab both sides
a²+b²+2ab-2ab=49-2ab
(a-b)²+2*15= 49 - 2*15
(a-b)²+30=49-30
(a-b)²=-11
(a-b)=√-11
In equation 1
a²+b²+2ab=49
a²+b²+2*15=49
a²+b²+30=49
a²+b²= 19
A/Q
a³-b³=(a-b)(a²+b²+ab)
put the values
a³-b³=√-11(19+15)
a³-b³= 24√-11