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LetA be a `2xx2`matrix with real entries. Let I be the `2xx2`identity matrix. Denote by tr (A), the sumof diagonal entries of A. Assume that `A^2=""I`.Statement1: If `A!=I`and `A!=""-I`, then det `A""=-1`.Statement2: If `A!=I`and `A!=""-I`,then `t r(A)!=0`. |
Answer» `A= [(a,b),(c,d)]` `A^2 = [(a,b),(c,d)][(a,b),(c,d)]` `= [(a^2+ bc, ab+bd),(ac+cd,bc+d^2)] = I` `a^2 + bc = bc + d^2 = 1` `ac+cd = ab+bd = 0` `c(a+d) = 0` `b(a+d) = 0` `c=0 or a=-d` not possible for c `b= 0 or a=-d` not possible for b `|(a,b),(c,d)| = ad - bc = -d^2 - bc` `= -(d^2 + bc) = -1` `tr (A) = a+d= a-a = 0` so, option 4 is correct |
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