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1.Algebra Booster

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Matrices and Determinants 7.79<br />

0 1<br />

where B = Ê ˆ<br />

Á<br />

Ë0 0 ˜<br />

¯<br />

Since B 2 = O, we get<br />

r<br />

B = O" r ≥2<br />

2005<br />

1 2005<br />

Thus, A = I + 2005B= Á<br />

Ë0 1 ˜<br />

¯<br />

51. Given,<br />

È1 0 0˘<br />

A =<br />

Í<br />

2 1 0<br />

˙<br />

Í ˙<br />

ÍÎ3 2 1˙˚<br />

fi |A| = 1 π 0<br />

fi A –1 exists<br />

Thus,<br />

Ê 1 0 0ˆ<br />

1<br />

A - = Á-2 Á<br />

1 0˜<br />

˜<br />

Ë 1 -2 1¯<br />

Ê1ˆ Ê 1 0 0ˆÊ1ˆ Ê 1ˆ<br />

Now,<br />

-1<br />

U1<br />

= A Á0˜ = Á- 2 1 0˜Á0˜ = Á-2˜<br />

Á ˜ Á ˜Á ˜ Á ˜<br />

Ë0¯ Ë 1 -2 1¯Ë0¯ Ë 1¯<br />

Ê2ˆ Ê 1 0 0ˆÊ2ˆ Ê 2ˆ<br />

-1<br />

U2<br />

= A Á3˜ = Á- 2 1 0˜Á3˜ = Á-1˜<br />

Á ˜ Á ˜Á ˜ Á ˜<br />

Ë0¯ Ë 1 -2 1¯Ë0¯ Ë-4¯<br />

Ê2ˆ Ê 1 0 0ˆÊ2ˆ Ê 2ˆ<br />

-1<br />

U3<br />

= A Á3˜ = Á- 2 1 0˜Á3˜ = Á-1˜<br />

Á ˜ Á ˜Á ˜ Á ˜<br />

Ë1¯ Ë 1 -2 1¯Ë1¯ Ë-3¯<br />

Ê 1 2 2ˆ<br />

(i) Thus, U = Á-2 -1 -1˜<br />

Á<br />

˜<br />

Ë 1 -4 -3¯<br />

1 2 2<br />

fi | U | =-2 -1 - 1 = 3<br />

1 -4 -3<br />

Ê 1 2 2ˆ<br />

(ii) Given, U = Á-2 -1 -1˜<br />

Á<br />

˜<br />

Ë 1 -4 -3¯<br />

fi<br />

Ê-1 -7 9ˆ<br />

1 1<br />

U - = Á-2 -5 6˜<br />

3Á<br />

˜<br />

Ë 0 -3 3¯<br />

Thus, the sum of the elements of U –1<br />

= 1 ( - 18 + 18) = 0<br />

3<br />

(iii) We have,<br />

È3˘<br />

[3 2 0] U<br />

Í<br />

2<br />

˙<br />

Í ˙<br />

ÍÎ0<br />

˙˚<br />

Ê 1 2 2ˆ Ê3ˆ<br />

= (3 2 0) Á-2 -1 -1˜ Á2˜<br />

Á<br />

˜ Á ˜<br />

Ë 1 -4 -3¯ Ë0¯<br />

Ê 7ˆ<br />

= (3 2 0) Á-8˜<br />

Á ˜<br />

Ë-5¯<br />

= (21 – 16) = (5)<br />

53. We have,<br />

1 -2 3<br />

D =-1 1 - 2 = 0<br />

1 -3 4<br />

and the determinant<br />

1 3 -1 0 1 k -1<br />

D1<br />

=-1 - 2 k =-1 - 2 k = 3-k<br />

1 4 1 0 3 k + 1<br />

Ê R1Æ R1+<br />

R2ˆ<br />

Á<br />

ËR Æ R + R ˜<br />

¯<br />

3 2 3<br />

If k π 3, D = 0, D π 0, therefore, the system of equations<br />

has no solutions.<br />

54. (i) Total number of matrices = 9 + 3 = 12<br />

We have<br />

Ê111ˆ Ê011ˆ Ê011ˆ<br />

A1= Á1 0 0 ˜, A2= Á1 1 0 ˜, A3=<br />

Á1 0 0˜<br />

Á ˜ Á ˜ Á ˜<br />

Ë100¯ Ë100¯ Ë101¯<br />

Ê1 0 1ˆ Ê0 0 1ˆ Ê0 0 1ˆ<br />

A4= Á0 0 1 ˜, A5= Á0 1 1 ˜, A6=<br />

Á0 0 1˜<br />

Á ˜ Á ˜ Á ˜<br />

Ë1 1 0¯ Ë1 1 0¯ Ë1 1 1¯<br />

Ê1 1 0ˆ Ê0 1 0ˆ Ê0 1 0ˆ<br />

A7= Á1 0 1 ˜, A8= Á1 1 1 ˜, A9=<br />

Á1 0 1˜<br />

Á ˜ Á ˜ Á ˜<br />

Ë0 1 0¯ Ë0 1 0¯ Ë0 1 1¯<br />

Ê1 1 0ˆ Ê010ˆ Ê010ˆ<br />

A10<br />

= Á1 0 1˜, A11 = Á1 1 1 ˜, A12<br />

= Á1 0 1˜<br />

Á ˜ Á ˜ Á ˜<br />

Ë0 1 0¯ Ë010¯ Ë011¯<br />

Alternate method: If two zeros are the entries in<br />

the diagonal, then<br />

3<br />

C 2<br />

¥ 3 C 1<br />

If all the entries in the principle diagonal is 1, then<br />

3<br />

C 1<br />

.<br />

Thus, the total number of matrices = 12.<br />

(ii) Here, |A 2<br />

| π 0, |A 3<br />

| π 0, |A 4<br />

| π 0<br />

and |A 5<br />

| π 0, |A 7<br />

| π 0, |A 9<br />

| π 0,<br />

Thus, there are six matrices A such that<br />

Êxˆ<br />

Ê1ˆ<br />

AÁy˜<br />

= Á0˜<br />

has a unique solution.<br />

Á ˜ Á ˜<br />

Ëz¯<br />

Ë0¯

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