已知，求下列各式的值:（1）； （2）； （3）.
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<table style="width:100%"> <tbody> <tr> <td> （1）∵<span mathtag="math"> <span> <span> a</span> <span style="vertical-align:super;font-size:90%"> <table style="display:inline-table;vertical-align:middle" cellpadding="-1" cellspacing="-1"> <tbody> <tr> <td style="border-bottom:1px solid black"> 1</td> </tr> <tr> <td> 2 </span> </span> +<span> <span> a</span> <span style="vertical-align:super;font-size:90%"> -<table style="display:inline-table;vertical-align:middle" cellpadding="-1" cellspacing="-1"> <tbody> <tr> <td style="border-bottom:1px solid black"> 1</td> </tr> <tr> <td> 2 </span> </span> =3</span> ，
∴<span mathtag="math" dealflag="1"> <span> <span> (<span> <span> a</span> <span style="vertical-align:super;font-size:90%"> <table style="display:inline-table;vertical-align:middle" cellpadding="-1" cellspacing="-1"> <tbody> <tr> <td style="border-bottom:1px solid black"> 1</td> </tr> <tr> <td> 2 </span> </span> +<span> <span> a</span> <span style="vertical-align:super;font-size:90%"> -<table style="display:inline-table;vertical-align:middle" cellpadding="-1" cellspacing="-1"> <tbody> <tr> <td style="border-bottom:1px solid black"> 1</td> </tr> <tr> <td> 2 </span> </span> ) </span> <span dealflag="1" style="vertical-align:super;font-size:90%"> 2</span> </span> </span> =a+a<sup> -1</sup> +2=9，
∴a+a<sup> -1</sup> =7；
（2）∵a+a<sup> -1</sup> =7，
∴（a+a<sup> -1</sup> ）<sup> 2</sup> =a<sup> 2</sup> +a<sup> -2</sup> +2=49，
∴a<sup> 2</sup> +a<sup> -2</sup> =47．
∴<span mathtag="math" dealflag="1"> <span> <span> (<span> <span> a</span> <span style="vertical-align:super;font-size:90%"> <table style="display:inline-table;vertical-align:middle" cellpadding="-1" cellspacing="-1"> <tbody> <tr> <td style="border-bottom:1px solid black"> 1</td> </tr> <tr> <td> 2 </span> </span> +<span> <span> a</span> <span style="vertical-align:super;font-size:90%"> -<table style="display:inline-table;vertical-align:middle" cellpadding="-1" cellspacing="-1"> <tbody> <tr> <td style="border-bottom:1px solid black"> 1</td> </tr> <tr> <td> 2 </span> </span> ) </span> <span dealflag="1" style="vertical-align:super;font-size:90%"> 2</span> </span> </span> =a+a<sup> -1</sup> +2=9，
∴a+a<sup> -1</sup> =7；
（2）∵a+a<sup> -1</sup> =7，
∴（a+a<sup> -1</sup> ）<sup> 2</sup> =a<sup> 2</sup> +a<sup> -2</sup> +2=49，
∴a<sup> 2</sup> +a<sup> -2</sup> =47．
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