Question

a + b + c = 7 and ab + bc + ca = 15, prove that a3 + b3 + c3 - 3abc = 28.​

Answer 1

Answer:

a³+b³+c³=(a+b+c)(a²+b²+c²-ab-bc-ac)

a²+b²+c²=49-30=19

a³+b³+c³=28

Answer 2

2

x−1

+2

x+1

=320

if x=4 then

2

4−1

+2

4+1

=320

2

3

+2

5

=320

40



=320

if x=5 then

2

5−1

+2

5+1

=320

2

4

+2

6

=320

80



=320

if x=6 then

2

6−1

+2

6+1

=320

2

5

+2

7

=320

160



=320

if x=7 then

2

7−1

+2

7+1

=320

2

6

+2

8

=320

320=320

Hence x=7