Suppose Appendix Table A.3 contained (z) only for z 0. Explain how you could still compute

Suppose Appendix Table A.3 contained ϕ(z) only for z ≥ 0. Explain how you could still compute
a. P(-1.72 ≤ Z ≤ - .55)
b. P(-1.72 ≤ Z ≤ .55)
Is it necessary to tabulate F(z) for z negative? What property of the standard normal curve justifies your answer?